CEMENT CHEMISTRY Harold F. W. Taylor
ACADEMIC PRESS London 1990
Cement Chemistry H. F. W. TAYLOR Emeritus Professor of Chemistry, University of Aberdeen Visiting Professor, Imperial College (University of London)
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Contents Preface 1
Portland cement and its major constituent phases
1
1.1 1.1.1 1.1.2 1.1.3
Introduction Portland cement: general Types of Portland cement Cement chemical nomenclature and other abbreviations
I I
1.2 1.2.1 1.2.2 1.2.3 1.2.4 1.2.5 1.2.6
Alite Polymorphism and crystal structure Tricalcium silicate solid solutions Compositions of alites in clinkers Polymorphic modifications of the alites in clinkers X-ray powder patterns and densities of tricalcium silicate and alites Optical, thermal and other data
1.3 1.3.1 1.3.2 1.3.3 1.3.4 1.3.5
Belite Polymorphism and crystal structure Lamellar textures in clinker belites Polymorphic types of belites in clinkers Compositions of belites in clinkers Cell parameters, X-ray powder patterns and other data
15 15 19 20 21
1.4 1.4.1
The aluminate phase Crystal structure: cubic, orthorhombic and monoclinic modifications Other modifications Structural modifications of the aluminate phase in clinkers Compositions of the aluminate phase in clinkers X-ray powder data, densities and optical properties
23
1.4.2 1.4.3 1.4.4 1.4.5
-1 4
5 5 8 8 9
13 15
-77
23 25 26 27 28
1S . 4
The ferrite phase 28 Crystal structure and con~positionin the Ca,(AI,Fe, _,),O, series 28 Compositions of the ferrite phase in clinkers 30 Crystal data and X-ray powder patterns for ferrite phase 31 containing foreign ions Optical, magnetic and other data 32
2
High-temperature chemistry
33
2.1
Introduction
33
2.2 2.2.1 2.2.2
Systems containing C a O with SiO, o r A1,0, or both The CaO-SiO, system The CaO-AI,O, system
33 33 34
1.5 1.5.i 1.5.2 1.5.3
vi
Contents
C,,A, and derived structures C,A,. C,A and C,A, The Ca0-AI,O,-St0, system Clinker formation In thc Ca0-AI,O,-St0,
systcm
Systems containing Fc,O, The CaO-Al,O,-Fe,O, system The Ca0-A120,-Fc20,-Si0, systcm Cl~nkerformation in the Ca0-AI,O,-Fe,03-SIO,
system
Systems containing MgO or FcO General Effect of MgO on equilibria in the Ca0-AI,O,-Fe,03-Si0, system Phases structurally related to gchlenite Systems containing alkalis or SO, or both Phases Equilibria Systems with other components Fluorides and fluorosilicates Carbonates Laboratory preparation of high-tempcrature phascs The chemistry of Portland cement manufacture General considerations Summary of the reactions in clinker formation Lime saturation factor, silica ratio and alumina ratio The Bogue calculation Enthalpy changes in clinker formation Raw materials and manufacturing processes Raw materials and fuel Dry and wet processes; energy requirements The dry process; suspension preheaters and precalciners The rotary kiln Circulation of volatiles; dust; cooling of clinker Other processes for clinker production; clinker grinding Reactions below about 1300'C Decomposition of carbonate minerals Decomposition of clay minerals and formation of products Sampling from cement kilns or preheater outlets Reaction mechanisms Condensation or reaction of volatiles Reactions at 1 300-1450•‹C Quantity of liquid formed
Contents
Burnabilities of raw mixes Nodulization Formation and recrystallization of alite Evaporation of volatiles; polymorphic transitions: reducing conditions
VII
79 81 81
83 84
Reactions during cooling, grinding or storage Solidification of the clinker liquid: indications from pure systems D o Portland cement clinkers contain glass or C,2A,'? Evidence from X-ray microanalysis Effects of cooling rate on the aluminate and ferrite phases Other effects of cooling rate Crystallization of the sulphate phases Changes during grinding or storage
84 85 86 87 88 89 91
Effects of minor components General Effects of s-block elements Effects of p- and d-block elements
92 92 93 94
Properties of Portland clinker and cement
96
Macroscopic and surface properties Unground clinker Part~clesize distribution of ground clinker o r cement Specific surface area determination Particle size distribution, phase composition and cement properties Chemical analysis
96 96 96 98
Light microscopy General Effects of bulk composition, raw feed preparation and ash deposition Effects of burning conditions and cooling rate Applications of light microscopic investigations Scanning electron microscopy, X-ray diffraction and other techniques Scanning electron microscopy X-ray diffraction Chemical or physical methods for separation of phases Other methods Quantitative phase composition General Calculation of quantitative phase composition from bulk analysis
99 100
101 10 1 103 104 104 105 105 108 11 1 112 113 113 113
.. .
VIII
Contents
Estimation of sulphate phases Estimation of major phases Limitations and modifications of thc calculation of phasc composition Comparison of rcsults of differcnt mcthods Reactivities of clinker phases Effect of major compositional variation Effects of ionic substitutions, defects and variation in polymorph Hydration of the calcium silicate phases Introduction Definitions and general points Experimental considerations Calcium hydroxidc Compositions of C3S and P-C,S pastes Calcium hydroxide content, thermal analysis and indircct determination of the Ca/Si ratio of the C-S-H Water contcnt of the C-S-H X-ray microanalysis and analytical electron microscopy Structural data for C3S and P-CIS pastes Microstructure Silicate anion structure X-ray diffraction pattern, densities and other data Structural models for C-S-H gel General 1.4-nm tobermoritc and jcnnitc C-S-H(1) and similar materials Products formed in suspensions from C,S or 0-C2S Structural relationships A mixed tobermorite-jennitc-type model for C-S-H gel Equilibria Solubility relations Species in solution Thermochemistry and thermodynamics Effects of alkalis Kinetics and mechanisms C,S: experimental data C3S: the initial reaction C,S: the induction pcriod The main reaction (C3S and P-C,S) Early hydration of P-C,S
Contents
Hydrated aluminate, ferrite and sulphate phases AFm phascs Compositional and structural principles The C,AH,, C,AC,,,H, and C,ACH, phases The C,ASH, phascs Other AFm phases containing aluminium AFm phases containing iron XRD patterns, thermal behaviour, optical propcrtics and IR spectra AFt phases Compositions and crystal structures Properties Other hydrated phases Hydrogarnet phascs CAH I, Brucite, hydrotalcite and related phascs Sulphate phases Equilibria and preparative methods The CaS0,-H20, CaS0,-Ca(OH),-H,O and CaS0,-K2S04-H,O systems The CaO-AI,O,-H,O, Ca0-A1,0,-Si0,--H20 and CaO-AI,O,-SO,-H20 systems Preparative methods Hydration reactions of the aluminate and fcrritc phases Reaction of C,A with water or with watcr and calcium hydroxide Reaction of C,A with watcr in the presence of calcium sulphate Reaction of the ferrite phase Enthalpy changes Hydration of Portland cement General description of the hydration process and products Evidence from X-ray diffraction Evidence from differential thermal analysis and infrared spectroscopy Evidence from light and electron microscopy Analytical data for cement pastes Determination of unreacted clinker phases Non-evaporable and bound water Thermogravimetry and determination of calcium hydroxide content
ix
x
Contents
Determinations of hydrated aluminate and silicatc phases Analyses of individual phases Silicate anion structure Interpretation of analytical data The nature of the ccment gel Fe,O,, SO, and minor oxide components in cement gel The stoichiometry of cement hydration Development of microstructure The early period of hydration The middle period of hydration The late period of hydration Calorimetry, pore solutions and energetics The early and middle periods Pore solutions after the first day Energetics of cement hydration Actions of calcium sulphate and of alkalis Setting Optimum gypsum Effects of alkalis Kinetics and modelling of the hydration process Kinetics: experimental data Interpretation of kinetic data Mathematical modelling of thc hydration process Structure and properties of fresh and hardened Portland cement pastes Fresh pastes Workability Rheology Models of fresh paste structure Hardened cement pastes: models of structure The Powers-Brownyard model Minimum waterlcement ratio for complete hydration; chemical shrinkage Calculation of volumetric quantities Later models of hardened paste structure Pore structure Porosities obtained by calculation Experimental methods: general points Determination of porosities by pyknometry Sorption isotherms; specific surface areas
Contents
Pore size distributions Mercury intrusion porosimetry (MIP) Other methods Strength Empirical relations between compressive strength and porosity Relations between strength and niicrostructurc or pore size distribution Mechanisms of failure Deformation Modulus of elasticity Drying shrinkage Creep Permeability and diffusion Permeability to water Diffusion of ions and gases
Composite cements Introduction Blastfurnace slag Formation, treatment and use in composite cements Factors affecting suitability for use in a composite cemcnt X-ray diffraction and microstructure of slags Internal structures of slag glasses Hydration chemistry of slag cements X-ray microanalysis Stoichiometry of slag cement hydration Activation of slag glasses Supersulphated cements Pulverized fuel ash (pfa; fly ash) low in CaO Properties Factors governing suitability for use in composite cements Rates of consumption of clinker phases and pfa, and contents of calcium hydroxide Microstructure and compositions of the hydration products The nature of the pozzolanic reaction Stoichiometry of pfa cement hydration Natural pozzolanas Properties Hydration reactions Microsilica (condensed silica fume)
xi
xii
Contents
Propcrties Hydration reactions Other mineral additions Class C fly ash Othcr pozzolanic or hydraulic additions Calcium carbonate and other fillcrs Pore structurcs and their rclation to physical propcrtics Porosities and porc size distributions Relations betwccn porc structure and physical propcrtics Calcium aluminate, expansive and other cements
Calcium aluminate cements Introduction Manufacture; chemical and mineralogical compositions Reactivities of the phases and methods of studying hydration Hydration reactions and products Thermodynamic calculations Setting times; mixing and placing Microstructural development Hardening; cffects of conversion Chemical admixtures Mixtures with calcite, slag, gypsum or Portland ccmcnt Reactions of calcium aluminate concrctc with external agents Refractory castables Expansive cements General Types of expansive cement Mechanism of expansion in Type K ccmcnts Other cemcnts Very rapidly hardening cemcnts Low-energy cements Alinite cements Admixtures and special uses of cements
Introduction Organic retarders and accelerators Retarders Mechanism of retardation Practical retarders Organic accelerators Air-entraining agents and grinding aids
Contents
Air-entraining agents Grinding aids Watcr reducers and superplasticizers Watcr reducers Supcrplasticizers Modc of action of water reducers and supcrplasticizers Zeta potential, rheology and nature of the sorbent phases Reasons for the enhanced dispersing power of superplasticizers Inorganic accelerators and retarders Accelerators of setting and hardening Modc of actlon Effects on the compositions and structures of the hydration products Precipitation effects; inorganic retarders and setting accelerators Effects of high or low temperatures at atmospheric pressure Hydration at 25-100•‹C Effects on kinetics and on the ultimate extent of hydration Effects on hydration products Low temperatures High-pressure steam curing General Basic chemistry of autoclave processes Mechanisms of reaction and equilibria Characteristics of hydrothermally formed C-S-H and tobermorite Oil-well cementing General Types of cement and of admixtures Effccts of temperature and pressure Very high strength cement-based materials General Phase compositions, microstructures and causes of high strength Concrete chemistry
Cement paste in concrete Portland cement mortars and concretes Backscattered electron imaging of the interfacial zone The nature of the paste-aggregate bond Composite cements and other topics Effects at exposed surfaces
xiii
xiv
Contents
Durability-general
aspccts
Carbonation, chloridc penetration and corrosion of rcinforcemcnt Gcncral Carbonation Transport and rcactions of chlorides Corrosion Alkali-silica reaction General Chemistry of alkali-silica reaction The expansion process ASR in mortars or concrctes made with composite cements Sulphatc attack General Calcium sulphate Magnesium sulphatc Effects of concretc properties and cement type Composite cements and sulphate attack Reactions involving sulphate and carbonate Other forms of attack Phys~calattack Leaching Miscellaneous forms of chemical attack Sea water attack Bacterial attack Miscellaneous paste-aggrcgate reactions Fire damage 409
References Appendix Index
Calculated X-ray powder diffraction patterns for tricalcium silicate and clinkcr phases
447 455
Preface This book deals with the chemistry of the principal silicate and aluminate cements used in building and civil engineering. It is a conlpletely re-written successor to the two volumes on 'The Chemistry of Cements' that I edited in 1964,* and, like them, is directed primarily to those whose background is in materials science or related disciplines. Emphasis is placed throughout on the underlying science rather than on practical applications, which are well covered in other works. The earlier volumes are outdated. Since 1964, several major areas previously unknown or neglected have been widely studied due to changes in the ways in which cement is made or used, o r to increased awareness of the problems encountered in producing concrete of adequate durability. Alongside this, much information has accrued from the introduction of new and powerful experimental techniques, of which scanning electron microscopy and X-ray microanalysis have proved especially important. In 1964, some topics, though practically important, were so imperfectly understood from the standpoint of basic chemistry that I did not consider it useful to deal with them. Increased understanding now makes it possible, and indeed essential, to include them. On the other hand, descriptions of standard experimental techniques, included in the earlier book, have been omitted, except for some matters of specialist relevance to cement chemistry. as good textbooks on all of them are available. All the cements considered in this book fall into the category of hydraulic cements; they set and harden as a result of chemical reactions with water, and if mixed with water in appropriate proportions continue to harden even if stored under water after they have set. Much the most important is Portland cement. Chapters 1 to 4 of the present work deal mainly with the chemistry of manufacture of Portland cement and with the nature of the resulting product. Chapters 5 to 8 deal mainly with the processes that occur when this product is mixed with water and with the nature of the hardened material. Chapters 9 to 1 I deal with the chemistry of other types of cement, of admixtures for concrete and o f special uses of cements. Chapter 12 deals with chemical and microstructural aspects of concrete, including ones relevant to processes that affect its durability o r limit its service life. The literature of cement chemistry is voluminous; the abstracting journal, Cements Rcsecrrch Progress has latterly listed about one thousand new contributions annually. Inevitably, coverage in the present work has been selective. I have tried to strike a balance between classical and recent contributions, in order to produce a book that will serve both as an * The Chenlistr~qf Cements (ed. H. F. W. Taylor), Vol. 1, 460 pp., Vol. 2. 442 pp.. Academic Press, London and New York (1964).
xvi
Preface
introduction that assumes no previous specialist knowledge of cements. and as a guide to research. It is hoped that the more important contributions up to early 1989 have been covered.
Acknowledgments
I am most grateful to former colleagues at the University of Aberdeen and friends elsewhere with whom I have had productive discussions over a period of many years. More directly in relation to the present book, Mr C. P. Kerton, D r G . R. Long, D r G . K. Moir and M r M. S. Sumner (Blue Circle Industries PLC) gave invaluable help, especially on the chemistry and technology of Portland cement production, composite cements, and clinker and cement properties. My thoughts on the hydration chemistry of calcium silicates owe much to discussions with Drs P. W. Brown, G. Frohnsdorfl'and H. M. Jennings, then all a t the National Bureau of Standards (now National Institute of Standards and Technology), USA. Those on microstructural aspects of the hydration of calcium silicates and cement similarly owe much to discussions with Professor P. L. Pratt and D r K . L. Scrivener, of Imperial College, London. Dr L. J. Parrott (British Cement Association) helped greatly with a discussion on the pore structures of cement pastes. Dr A. Capmas (Lafarge Fondu International, France) and Drs D. MknetricrSorrentino and F. Sorrentino (Lafarge Coppee Recherche, France) gave quite exceptional help in providing a draft manuscript on calcium aluminate cements and with subsequent discussions, in which D r C. M. George (Lafarge Calcium Aluminates, USA) also participated. D r J. Bensted (The British Petroleum Company PLC) corrected some important errors on oil well cementing. For discussions and comments on various aspects of concrete durability, I am most grateful to M r J. S. Lumley and Dr J. J. Kollek (Blue Circle Industries PLC). Many of those mentioned above gave help that included comments on parts of the manuscript, but any errors are mine. Many people, noted in the reference list, generously made results available in advance of publication. M r A. M. Harrisson and Mr N. B. Winter ( W H D Microanalysis Consultants, Ltd), D r D. C. Pomeroy (British Cement Association), D r D. Menttrier-Sorrentino and Dr K . L. Scrivener kindly provided some excellent electron or light micrographs, noted in the respective captions. Mr C. P. Kerton freely gave extensive bibliographical assistance, and I thank M r S. Black and Ms J. Kerr (University of Aberdeen) for photography of the line drawings. I thank publishers (as copyright holders; italics) and authors for permission to
Preface
xv~i
reproduce the figures noted below. Authors and sources, including copyright years. are given in the captions and reference list, and where it was requested, additional information is included below. Atncricun Cerrrn~icSoc.ic~,v,Fig. 5.4C and D (Ref. J 10. Issue 10, Oct., on "Morphological development of hydrating tricalcium silicate as examined by electron microscopy techniques"); Fig. 5.6 (Ref. M38, lssue 12. Dec., on "Analytical electron microscopy of cement pastes: IV. p-dicalcium silicate pastes"); Figs. 5.8 and 5.9 (Ref. T24, lssue 6, June, on "Proposed structure for calcium silicate hydrate gel"); Fig. 5.12 (Ref. J 15, lssue 8, Aug., on "Aqueous solubility relationships for two types of calcium silicate hydrate"); Fig. 9.3 (Ref. T44, lssue 12, Dcc., on "Analytical study of pure and extended Portland cement pastes: 11. fly ash- and slagcement pastes"). A n ~ e r i c ~Concrete ~n Inslitute, Fig. 1 1.6. Atwriccrn Jourtltr/ c!f'Scicr~c,c. Fig. 6.5. Britislr C~vnentAssociution. Fig. 5.1 1. Cemento, Fig. 1 1.4. D~otorland RILEM, Figs. 5.10 and 5.13. Edirions Seprinur, Fig. 10.7. Elscvier Applic~lSc~ic~nw Publishers Ltd, Figs. 10.1, 10.6 and 10.8. Intcrntrtionrrl C c w m t Microsc,op!l As.sociation, Figs. 4.3A, 4.3B and 7.2. John Wiley andsons Ltd, Fig. 8.4. Mr~terirrlsRcsrtrrc~h Society, Figs. 8.7, 9.1, 9.2, 9.6 and 12.1. Pulludiut~ Puh1icution.s Ltrl, Fig. 7.1. Pergamon Press PLC, Figs. 1.2, 3.3, 5.3, 5.5, 5.14, 5.15, 8.8, 11.1, 11.5and 12.5. Tlrc Royrd Socic~ty,Fig. 7.7. SirIdi~(ll0Nuc.ionrr1 ckr IndListriu do Cinwnto (Brazil), Fig. 6.8. Societj, of'Cl~c~tnicd Itr~/ustry,Fig. 5.1. Stroyizdut, Fig. 3.4. Tlromu.s Telfird Ltd, Figs. 12.4 and 12.6. Tronsporltrtion Reserrrch Board, Nrrtiot1r11 Rrsetrrc.11 Council, Wus11itigton DC, Fig. 7.8 (Ref. L32. on "Changes in composition of the aqueous phase during hydration of cement pastcs and suspensions").
Portland cement and its major constituent phases
1 .I
1 .I . I
Introduction
Portland cement: general
Portland cement is made by heating a mixture of limestone and clay. or other materials of similar bulk composition and sufficient reactivity, ultimately to a temperature of about 1450•‹C.Partial fusion occurs, and nodules of clinker are produced. The clinker is mixed with a few per cent of gypsum and finely ground to make the cement. The gypsum controls the rate of set and may be partly replaced by other forms of calcium sulphate. Some specifications allow the addition of other materials at the grinding stage. The clinker typically has a composition in the region of 67% CaO, 22% S O 2 , 5% AI,O,, 3 % Fe,O, and 3% of other components, and normally contains four major phases, called alite, belite, aluminate phase and ferrite phase. Several other phases, such as alkali sulphates and calcium oxide. are normally present in minor amounts. Alite is the most important constituent of all normal Portland cement clinkers, of which it constitutes 50-70%. It is tricalcium silicate (Ca,SiO,) modified in composition and crystal structure by incorporation of foreign ions, especially M g Z C , A13+ and Fe3+. It reacts relatively quickly with water, and in normal Portland cements is the most important of the constituent phases for strength development; at ages up to 28 days, it is by far the most important. Belite constitutes 15-30% of normal Portland cement clinkers. It is dicalcium silicate (Ca,SiO,) modified by incorporation of foreign ions and normally present wholly or largely as the P polymorph. It reacts slowly with water, thus contributing little to the strength during the first 28 days. but substantially to the further increase in strength that occurs at later ages. By one year, the strengths obtainable from pure alite and pure belite are about the same under comparable conditions. The aluminate phase constitutes 5-10% of most normal Portland cement clinkers. It is tricalcium aluminate (Ca3AIz0,), substantially modified in
2
Cement Chemistry
composition and sometimes also in structure by incorporation of foreign ions. especially Si4+, Fe", N a f and K + . I t reacts rapidly with water. and can cause undesirably rapid setting unless a set-controlling agent, usually gypsum, is added. The ferrite phase makes up 5 15% of normal Portland cement clinkers. I t is tetracalcium aluniinoferrite (Ca,AIFeO,) substantially modilied in cornposition by variation in AI/Fe ratio and incorporation of foreign ions. The rate at which i t reacts with water appears to be somewhat variablt.. perhaps due to ditTerenccs in composition or other characteristics, but in general is high initially and intermediate between those ol'alite and belite a t later ages.
1 .I .2 Types of Portland cement
The great majority of Portland cements made throughout the world are designed for general constructional use. The specifications with which such cements must conlply are similar, but not identical. in all countries and various nanies are used to define the material, such as OPC (Ordinary Portland Cement) in the U K , or Type I Portland Cement in the USA. Throughout this book, we shall use the term 'ordinary' Portland cements to distinguish such general purpose cements from other types of Portland cement, which are made in snialler quantities for special purposes. Specifications are, in general, based partly on chemical composition or physical properties such as specific surface area. and partly on performance tests, such as setting time o r compressive strength developed under standard conditions. The content of MgO* is usually limited to either 4 or 5% because quantities of this component in excess of about 2% are liable to occur as periclase (magnesium oxide), which through slow reaction with water can cause destructive expansion of hardened concrete. Free lime (calcium oxide) can behave similarly, and its potential formation sets a practical upper limit to the alite content of a clinker; in UK specifications, this is controlled by a compositional parameter called the lime saturation factor (LSF), which is discussed in Sections 2.3.3 and 3.1.2. Excessive contents of SO, can also lead to delayed expansion, and upper limits of 2.54% are usually imposed. Alkalis (K,O and N a 2 0 ) can undergo expansive reactions with certain aggregates, and some national specifications limit the content, e.g. to 0.6% equivalent N a 2 0 ( N a 2 0 + 0.66K20). Other upper limits of composition widely used in specifications relate to matter insoluble *Confusion can arise because the names or formulae of compounds c u i he used to denote either phases or components; this applies espec~allyto C a O and MgO. Here and elsewhere, we shall generally use chemical or mineral names of oxides (e.g. calciuni oxide. magnesium oxide. lime, periclase) for phases and formulae (e.g. CaO, MgO) Tor components. Mineral names or prefixed formulae (e.g. a-Al,O,) are never used for components.
Portland Cement and its Major Constituent Phases
3
in dilute acid, and loss on ignition. Many other minor components are limited in content by their effects on the manufacturing process, o r the properties, or both, and in some cases the limits are defined in specifications. Rapid-hardening Portland cements have been produced in various ways, s ~ l as ~ hvarying the composition to increase the alite content. f n e r grinding of the clinker, and improvements in the manufacturing process. e.g. finer grinding or better mixing of the raw materials. The alite contents of Portland cements have increased steadily over the one and a half centuries during which the latter have been produced, and many present-day cements that would be considered normal today would have been described as rapid hardening only a few decades ago. In USA specifications, rapid-hardening Portland cements are called high early strength or Type Ill cements. Destructive expansion from reaction with sulphates can occur not only if the latter are present in excessive proportion in the cement, but also from attack on concrete by sulphate solutions. The reaction involves the AI,O,containing phases in the hardened cement, and in sulphate-resisting Portland cements, its effects are reduced by decreasing the proportion of thc aluminate phase, sometimes to zero. This is achieved by decreasing the ratio of A120, to Fe,O, in the raw materials. In the USA, sulphate-resisting Portland cements are called Type V cements. White Portland cements are made by increasing the ratio of A1,0, to Fe20,, and thus represent the opposite extreme in composition to sulphatcresisting Portland cements. The normal, dark colour of Portland cement is due to the ferrite phase, formation of which in a white cement must thus be avoided. It is impracticable to employ raw materials that are completely free from F e 2 0 , and other components, such as Mn,O,, that contribute to the colour. The effects of these components are therefore usually minimized by producing the clinker under slightly reducing conditions and by rapid quenching. In addition to alite, belite and aluminate phase, some glass may be formed. The reaction of Portland cement with water is exothermic, and while this can be an advantage under some conditions because it accelerates hardening, it is a disadvantage under others. such as in the construction of large dams or in the lining of oil wells, when a cement slurry has to be pumped over a large distance under pressure and sometimes a t a high temperature. Slower heat evolution can be achieved by coarser grinding, and decreased total heat evolution by lowering the contents of alite and aluminate phase. Specifications in the USA include definitions of a Type 11 or 'moderate heat of hardening' cement, and a more extreme Type IV or 'low heat' cement. The Type 11 cement is also suitable for general use in constructions exposed to moderate sulphate attack. Heat evolution can also be decreased by partially replacing the cement by pfa (pulverized fuel ash; fly ash) or other materials
4
Cement Chemistry
(Chapter 9), and this is probably today a more usual solution. The specialized requirements of oil well cements are discussed in Section 11.8.
1.I .3 Cement chemical nomenclature and other abbreviations
Chemical formulae in cement chemistry are often expressed as a sum of oxides; thus tricalcium silicate, Ca,SiO,, can be written as .?CaO.SiO,. This does not imply that the constituent oxides have any separate existence within the structure. I t is usual to abbreviate the formulae of the conmoner oxides to single letters, such as C for CaO or S for SiO,. Ca,SiO, thus becoming C,S. This system is often combined with orthodox chemical notation within a chemical equation, e.g. 3Ca0
+ SiO,
+
C,S
(1.1)
or even within a single formula, as in C , ,A,CaF2 for Ca12AI,,0,,F2. The abbreviations most widely used are as follows: C = CaO M=MgO T = TiO,
S = SiO, K=K,O P = P,O,
A = A1,0, S = SO, H = H,O
-
F = Fe,O, N = Na,O C = CO,
The formulae of the simple oxide phases (e.g. CaO) are usually written in full. Other abbreviations and units used in this book are as follows: Techniques
BE1 = backscatterd electron imaging. BET method = Brunauer-EmmettTeller. DTA = differential thermal analysis. EPMA = electron probe microanalysis. ESCA = electron spectroscopy for chemical analysis (X-ray photoelectron spectroscopy). GLC = gas-liquid chromatography. GPC = gel permeation chromatography. IR = infrared. MIP = mercury intrusion porosimetry. NMR = nuclear magnetic resonance. QXDA = quantitative X-ray diffraction analysis. SEM = scanning electron microscop(e,y). STEM = scanning transmission electron microscop(e,y). TEM = transmission electron microscope(e,y). T G = thermogravimetry. TMS = trimethylsilyl(ation). XRD = X-ray diffraction. XRF = X-ray fluorescence. Materials
C-S-H = poorly crystalline or amorphous calcium silicate hydrate of unspecified composition. Ggbfs = ground granulated blast furnace slag. Hcp = hardened cement paste. Pfa = pulverised fuel ash (fly ash).
Portland Cement and its Major Constituent Phases
5
properties or reactions
AR = alumina ratio (alumina modulus). ASR = alkali silica reaction. LSF = lime saturation factor. SR = silica ratio (silica modulus). C, = analytical (total) concentration of x, irrespective of species. [x] = concentration of species x. {x} = activity of species x. RH = relative humidity. Na,O, = equivalent N a 2 0 (wt % Na,O + 0.66K20). (+)2V, (-)2V = optic sign and optic axial angle. Pressure units
I MPa
=
I Nrnm-,
=
lobar
=
9.87atm = 7500torr
=
145.0Ibin-~.
1.2 Alite
1.2.1 Polymorphism and crystal structure On being heated, pure C,S undergoes a series of reversible phase transitions, which have been detected by a combination of DTA, high-temperature X R D and high-temperature light microscopy (BI,G I ,Y I ,RI .M ILM5): 620 C
920•‹C
980•‹C
990•‹C
1060'C
T , ~ T z - T j . - M I - - M 2 ~ ~ R (T = triclinic, M = monoclinic, R = rhombohedral).
1070 C
The pure compound, when cooled to room temperature, is thus T , . In production clinkers, due to the incorporation of foreign ions, the form present at room temperature is normally M , or M,, or a mixture of these; rarely, T, is found (MILM5,TI). There has been some uncertainty as to the number and nomenclature of these polymorphs; reported M I , and M2, forms appear to be identical with M,, leaving reported M,, and M,, forms to be called simply M, and M,, respectively (M4,M5). Jeffery ( J l ) made the first determination of the crystal structure. He showed that the forms now known as R, T I and M, had closely similar structures, and determined the approximate or pseudostructure common to all three; it was built from Ca2", SiOl- and 02-ions, the latter being bonded only to six Ca2" ions, as in CaO. Later, more exact determinations were reported for T , (G2),M, stabilized by Mg2+ (NI), R at 1200•‹C(N2) and R stabilized with Sr2+ (11). Fig. 1.1 shows the structure of the R form. The known structures are all closely similar as regards the positions of the Ca2" and 0,- ions and of the silicon atoms, but differ markedly in the orientations of the Si0:- tetrahedra, which in some cases are disordered.
6
Cement Chemistry
Fig. 1.1 Crystal structure of the R modification of C,S, based o n thc results of Nishi and Takeuchi (N2) and showing calcium atoms (large. open circles). silicon atoms (small, open circles), oxide ions (large. hatched circle) and oxygen tetrahedra (triangles). Heights of atoms are in thousandths of the cell height ( c = 2.5586 nni). slashes denoting statistical alternatives. Oxygen tetrahedra are shown in averaged orientations; in reality. each is tilted around its enclosed silicon atom statistically in any one of three directions so as to preserve the threefold symmetry. All tetrahedra point upwards excepting those surrounding the silicon atoms a t height 213. of which 70% point downwards and 30% upwards. Only the bottom third of the cell is shown. the middle and top thirds being derivable from it by translations o r 113. 2 3, 1 :3 and 2 3, 113, 213 parallel to the o, h and c axes, rcspectivcly.
The structural differences between the polymorphs affect the coordination of the Ca2+ ions and the oxygen atoms of the SiOi- tetrahedra. For each polymorph, there are several crystallographically distinct calcium sites, having different coordination, and for a given site, the coordination sometimes varies between individual atoms due to orientational disorder in the surrounding Si0:tetrahedra. Definitions of the calcium coordination numbers are somewhat arbitrary due to variations in the lengths of the bonds; for example, in the R form at 1 200•‹C,the calcium atoms in one of the sites could be regarded as 7 coordinated if bonds as long as 0.296 nm are counted, and 5 coordinated if they are not (N2). If such abnormally long bonds are excluded, the mean coordination number of the calcium is 5.66 in the R polymorph, 6.15 in M, and 6.21 in T, (M5). In relation to reactivity towards water, the coordination of the oxygen atoms is possibly more important than that of Ca. This has not been discussed in detail in the literature, but mean oxygen coordination numbers may be expected to increase with those of calcium.
Table 1.1 Crystal data for the C,S polymorphs
Polymorph Pseudostructure R (at 1200•‹C) R (stabilized with Sr) M, (stabilized with Mg) TI
Space group R3m R3m R3m
c!? PI
Cell parameters (a,b,c in nm)
b
7
-
-
-
-
120" 120" 120"
a
b
c
a
0.70 0.7135 0.70567 3.3083 1.167
-
2.50 2.5586 2.4974 1.8499 1.372
0.7027 1.424
-
105.5"
-
94.12' 94.3"
-
90.0"
Z
Ref.
9 9 9 36 18
Jl N2 11 N1 G2
8
Cement Chemistry
Table I . I gives those crystal data for the C3S polymorphs that have been obtained using single crystal methods. The literature contains additional unit cell data. based only on powder diffraction evidence. Some of these may be equivalent to ones in Table 1 . 1 , since the unit cell of a monoclinic or triclinic crystal can be defined in different ways, but some are certainly incorrect. Because only the stronger reflections are recorded, and for other reasons. it is not possible to determine the unit cells of these complex structures reliably by powder methods. The unit cells of the T I . M, and R forms are superficially somewhat different, but all three are geometrically related; transformation matrices have been given (12,Hl).
1.2.2 Tricalcium silicate solid solutions Hahn ct ul. (H2) reported a detailed study of the limits of substitution in C,S of MgO, AI,O, and Fe,O,, alone and in combination. For MgO. the limit was 2.0% at 1550"C, falling with temperature to 1.5% at 1420•‹C. The substitution was of MgZf for Ca2'. For A1,0,, the limit was 1.0%. irrespective of temperature, and the substitution was partly of 2 Al" for C a 2 + + Si4+ and partly of 4 A13+ for 3 Si4+ and a vacant octahedral site. For Fe,O,, the limit was 1 . I Oh, and substitution was of 2 Fe3 for C a 2 + + Si4+. The limit of substitution for MgO was not affected by the incorporation of A13+ or Fe3+ or both, nor vice versa, but A13 and Fe" competed for the same sites and the limit for one was decreased in the presence of the other. Incorporation of any of these oxides in sufficient quantity caused the higher temperature polymorphs to persist on cooling to room temperature. For each combination of substituents studied, increase in the total proportion of substituents led to the persistence of successively higher temperature polymorphs in the sequence T , -+T, -+ M I . In the system CaO-MgOA1,0,-SiO,, total contents of impurity oxide of about 1 % caused T I to be replaced by T,, and ones of about 2% caused T, to be replaced by M I . Slightly higher total impurity contents were needed to effect each of these changes if Fe20, was present. Woermann et al. (WI) found that up to 1.4% of either N a 2 0 or K,O could be incorporated into C,S at 1500•‹Cand discussed mechanisms of substitution. +
+
1.2.3 Compositions of alites in clinkers
Many workers have analysed alites from ordinary production clinkers by Xray microanalysis (T2,T3,K 1 ,G3,B2,G4,Sl ,H3,H4) or chemical analysis of
Portland Cement and its Major Constituent Phases
9
separated material (YI,U I ) ; alites from white cement clinkers have also been analysed (B3). Considering the errors inherent in either method, the agreement between the various sets of data is generally good. The most important variations are in the contents of CaO, MgO and Fe,O,. Table 1.2 g''ives an average composition, with corrections to the contents of these components based on considerations given in this and the previous sections. Pure C,S contains 73.7% of CaO and 26.3% of SiO,. Alites in clinkers typically contain 3-4% of impurity oxides, i.e. oxide components other than those present in the pure compound. Boikova (B4) found a positive correlation between the total percentage of impurity oxides in the alite (I;,)and the percentage of MgO in the clinker (M,). Her results are approximately fitted by the equation I, = 0.7M, + 2.1. Correlations have also been found between the percentages of individual impurity oxides in the alite and in the clinker. In the following discussion, we shall use symbols of the type 0, and 0, to denote these quantities, where 0 is the cement chemical symbol for the oxide and the suffixes a and c indicate alite and clinker, respectively, For MgO, Kristmann's ( K I ) data indicate the approximate relations M a = 0.67 x M , for M , 6 3.0 and M a 2.0 for M , > 3.0. Yamaguchi and Takagi (YI) and Terrier (T2) found similar correlations, but with slopes of 0.74 and 0.45, respectively. For A1,0,, Kristmann (KI) did not find any significant correlation, but for Fe,O, his data indicate the approximate relation Fa = 0.33 x F, for 1;, < 4%. Boikova (B4) reported ratios of 0.34 to 0.82 for M J M , , 0.09 to 0.23 for A , / A , and 0.17 to 0.36 for FJF,. Several authors have discussed the types of substitution that occur in clinker alites (GI,G3.H3,B4). In general, the evidence as regards limits and patterns of substitution is consistent with the conclusions of Hahn c l crl. (H2) given in Section 1.2.2, but the contents of K 2 0 and N a 2 0 are much below the limits found by Woermann et al. (Wl). Table 1.3 includes atomic ratios corresponding to the typical composition in Table 1.2, assuming 1.1% MgO. The cations are arranged in approximate sequence of ionic radius; in the absence of precise information from X-ray structure determinations, this, taken in conjunction with a knowledge of the known preferences of ions as regards coordination number, appears to provide the most rational basis for suggesting allocations of atoms to sites, and one that is consistent with the compositional data, both for alite and for the other clinker phases.
-
1.2.4
Polymorphic modifications of the alites in clinkers
Maki and co-workers (M2-M6) studied the crystallization of alite by hightemperature light microscopy of thin sections together with thermal and X-
Table 1.2 Typical compositions of phases in Portland cement clinkers (weight per cent)
Alite Belite Aluminate (cubic) (orthorhombic) (low Fe) Ferrite Ferrite (low Al)
Note
Na,O
1 2 3 4 5 6 7
0.1 0.1 1.0 0.6 0.4 0.1 0.1
MgO A120, 1.1 0.5 1.4 1.2 1.0 3.0 2.8
1.0 2.1 31.3 28.9 33.8 21.9 15.2
SiO,
P,O,
SO,
K20
CaO
TiO,
Mn,O,
Fe20,
25.2 31.5 3.7 4.3 4.6 3.6 3.5
0.2 0.2 0.0 0.0 0.0 0.0 0.0
0.0 0.1 0.0 0.0 0.0 0.0 0.0
0.1 0.9 0.7 4.0 0.5 0.2 0.2
71.6 63.5 56.6 53.9 58.1 47.5 46.0
0.0 0.2 0.2 0.5 0.6 1.6 1.7
0.0 0.0 0.0 0.0 0.0 0.7 0.7
0.7 0.9 5.1 6.6 1 .O 21.4 29.8
Notes 1. For a clinker with 1.65% MgO and 2.6% Fe20,. More generally. % MgO =0.667 x % MgO in clinker. but not >2.0%; OO/ F e 2 0 , =0.25 x % Fe20, in clinker, but not > 1. I %; % CaO = 71.6 - 1.4 x (% MgO in alite - 1.I). The composition should be normalized to a total of 100%. 2. SO, higher in clinkers of high S 0 , , ( N a 2 0 + K 2 0 ) ratio. 3. Normal form in clinkers low in alkali and with A120,:Fe20, in the approximate range 1.G3.0. 3. Orthorhombic or pseudotetragonal forms, found in clinkers high in alkalis. Na,K ratio varies with that of the clinker. 5. Tentative composition for aluminate phase in white cement clinkers. 6. Normal composition for clinkers with AI2O,iFe2O, in the approximate range 1.0 3.0. 7. Tentative composition for ferrite phase in sulphate-resisting clinkers.
Table 1.3 Atomic ratios for phases in Portland cement clinkers, calculated from the typical compositions in Table 1.2
K
Belite Aluminate (cubic) Aluminate (orthorhombic)" Aluminate (low Fe) Ferrite -
~
Ferrite (low Al)
" See notes to Table 1 .I
Na
Ca
Mg
Mn
Ti
Fe
Al
Si
P
0
12
Cement Chemistry
ray methods. It crystallizes from the clinker liquid at about 1450 C as the R polymorph, which inverts to lower temperature forms on cooling. Due to the lowering of symmetry, each crystal ~ ~ s u a lcontains ly several optically distinct domains, recognizable in thin section using polarized light. At the higher temperatures of formation or recrystallization within the normal range, small crystals, relatively high in foreign ions, are formed, which on cooling invert to M,. Crystals formed at lower temperatures are larger and poorer in foreign ions, and may undergo further transition, partial or complete. to M I or, rarely, to T,. The MgO and SO, contents of the clinker are especially important in determining whether the transformation into M I occurs. High MgO contents favour the formation of small crystals that invert to M,, and high SO, contents that of large crystals that invert further to M I (Fig. 1.2). though the tendency to form M , is decreased if the ratio of alkalis to SO, is high. as the SO, is then largely combined as alkali sulphates. Crystals containing both M, and M I can occur, and are often zoned, with cores of M I and peripheral regions of M,. This is because the clinker liquid becomes richer in MgO us crystallization proceeds, causing the material deposited later to persist as M,. The M, to M I transformation is also atyected by cooling rate. slow cooling favouring the transformation to M I . In certain slowly cooled clinkers. further transformation may occur, giving T,. This is likely to occur only if the alite is particularly low in substituents.
-
1.o 2.0 SO3 in Clinker (%) Fig. 1.2 Dependence of the polymorphic modification of alite in production clinkers on the MgO and SO, contents of the clinker. Maki and Goto (M6). O0
Portland Cement and its Major Const~tuentPhases
13
On prolonged heating a t high temperatures, the large alite crystals grow at the expense of smaller ones, and in this process the content of foreign ions decreases, favouring inversion of M, to M I or T,. Production clinkers are usually sufficiently high in MgO to prevent this inversion from taking place. even though considerable recrystallization may occur (M6). Rccrystalliziition can thus markedly affect the relation between temperature of Sormntion and crystal size. Guinier and Regourd ( G I ) concluded from X R D evidence that the R polymorph occurred in certain clinkers. This view has been acccptcd by many other workers, but Maki and Kato (M4) concluded from thermal and other evidence that the supposed R form was really M ,. which had been misidentified because of the similarity of its X R D powder pattern to that of the R form. They gave two strong reasons for supposing thc alites in question to be M , and not R. Firstly, they contained less MgO than M , alites; for R to persist at room temperature, it would have to contain more MgO than M,, because it is a higher temperature form. Secondly. phase transitions were detected on heating; R, again because i t is the hightemperature form, would not undergo these. Sinclair and Groves (S2) also described a case of the misidentification of an alite as rhombohedral; using single crystal electron diffraction, they found that some laboratory-prepared alites that had been regarded as rhombohedral were really either nionoclinic or triclinic. On present evidence, it is thus probable that the only alite polymorphs found in normal production clinkers are M,, M , and, occasionally, T2, itnd that those that have been described as R are really M However, both X R D and thermal evidence show that some alites formed in the presence of fluoride are rhombohedral (M7). X R D and optical evidence indicates that the commonest polymorph in production clinkers is almost certainly M,. the presence of which in such a clinker has also been demonstrated using single crystal electron diffraction (H 1).
,.
1.2.5
X-ray powder patterns and densities of tricalciurn silicate and alites
A number of experimentally determined X R D powder patterns have been reported for C,S and alite polymorphs (PI), but because of uncertainties in the indexing and sometimes in identification of the polymorph, these are usefully supplemented by patterns calculated from the crystal structures (Appendix). The patterns of the polymorphs are closely alike, and in clinkers, interpretation is complicated by the fact that many peaks overlap ones of other phases. The pattern depends not only on the polymorph or
14
Cement Chemistry
mixtures of polymorphs present, but also on the nature and amounts of the foreign ions present in each. It is therefore not always easy or even practicable to identify the polymorph or polymorphs in a clinker by the XRD powder pattern alone. The patterns differ principally in the fine structures of certain peaks (JI ,G I ,RI ,M4,M5,B4). Fig. 1.3 shows characteristic portions of the patterns of T , C3S and of some clinkers. M , alite, as present in clinkers, probably always has the maximum MgO content possible for this polymorph, and as such has a unit cell o r pseudocell that is geometrically almost hexagonal. This causes the peak at about 51.7" 20 (CuK, radiation) to be almost a singlet; the corresponding peak of the M , alite is a well-defined doublet, and those of T, C3S and T, alite are triplets. There are also differences in the peaks at 32" to 33" 28, but these are less useful because of overlaps with other phases. Because of their high relative intensities, care is needed with these latter peaks to avoid intensity errors from extinction.
DEGREES 2 0 (Cu K a ) CJla 4
'to
4 04 4
- 4
2"
D E G R E E S 2 0 (Cu Ka) Fig. 1.3 Portions of XRD powder patterns of (A) the T I modification of C3S at 605•‹C(after Regourd (R2)); (B), (C) and (D) clinkers containing, respectively, M,, (M, M,) and M , alites (after Maki and Kato (M4)). Indexing of T I and M3 patterns is based on the axes in Table 1.1 and calculated intensities.
+
Portland Cement and its Major Constituent Phases
15
The X-ray density of T I C,S is 3 1 2 0 k g n 1 - ~ ( ( 3 2 ) ; that of M, alite, calculated assuming the atomic parameters of Nishi pt crl. ( N I ) and the is 3 150 composition (Cao.,8Mgo.o,A~o.oo,,Feo.oo33~3~Sio.9,A~o.030s kg m-,. 1.2.6 Optical, thermal and other data
T , C,S is biaxial with (-)2V = 20-6O0, and a > c,,, = 0-15" (01); the refractive indices are a = 1.7 139, y = 1.71 72 for pure C,S and a = 1.7 158 -1.7197, y = 1.7220-1.7238 for typical clinker alites, all determined using sodium light (B5). In thin sections in polarized light, M , and M, are most readily distinguished by the maximum birefringence, which is 0.005-0.006 for M, and 0.003 for M I (M2-M5). Other differences in optical properties include the optic orientation referred to the hexagonal axes of the R form, the shape of which persists through the transitions that occur on cooling, and the optic axial angle; Maki and Kato (M4) give details. It is doubtful whether the polymorphs can be reliably distinguished by DTA, since both the M I and the M, forms are metastable at low temperatures and transform to T, a t about 700•‹C(M4,M5). This effect is normally swamped by the endothermic to a', transition of the belite. Due to the effect of substituents. neither T, nor M, appears to form on heating, and endothermic transitions occur at 790-850"C, giving M , , and at 950•‹C. giving M,. These could occur irrespective of the form initially present; however, clinkers high in MgO, in which the alite is present entirely as M,, normally show no thermal effects attributable to the alite until more than 1000•‹C.when transition to R occurs. A substituted C,S has been found in nature and given the mineral name hatrurite ((35). The rock containing it is interesting because it contains other phases present in clinker and appears to have been formed under conditions somewhat similar to those existing in a cement kiln. 1.3
Belite
1.3.1 Polymorphism and crystal structure Thermal and X-ray evidence (RI ,N3,B6) show that five polymorphs of C,S exist at ordinary pressures, viz:
a (H
=
high; L
1425•‹C
=
low)
'
1 160 C
63&680JC
, 7 Py-
i 500
690•‹C 780-860•‹C
C
I
16
Cement Chemistry
The structures of all of them are built from C a 2 + and Si0,'- ions. The arrangements of these ions are closely similar in the a , a',. a',, and P polymorphs, but that in y-C,S is somewhat difrerent. As with C,S, the higher temperature polymorphs cannot normally be preserved on cooling to room temperature unless stabilized by foreign ions. y-C2S is much less dense than the other polymorphs, and this causes crystals or sintered masses of PC,S to crack and fall to a more voluminous powder on cooling. a phenomenon known as dusting. If the crystallites of P-C2S are sufficiently small, the transforn~ationdoes not occur, even if no stabilizer is present (Y3).In all normal Portland cement clinkers, the belite contains enough stabilizing ions to prevent the transformation to y-C,S from taking place. Many of the earlier crystal data reported for the C2S polyniorphs were based on powder X R D patterns obtained either at high temperatures or from stabilized preparations at room temperature. As with the C,S polymorphs, some such data have subsequently been shown to be incorrect. and the remainder must be considered unreliable. Table 1.4 gives data derived from single crystal evidence, which does not suffer from this limitation. The structures of the u, a',, a',, and f3 poly~~iorphs (Fig. 1.4A C) all belong to a large family typified by that of glascrite, K,Na(SO,), (MX). As is apparent from the data in Table 1.4, the n nit cells of all four arc closely related. High-temperature single crystal X-ray patterns of a-CIS show symmetry consistent with the space group P6,/mmc. but U d a p w ; ~ct (11. (U6,U7) found that this was due to the presence of a domain structure, individual domains being structurally similar to glascrite. for which the space group is ~ 3 1 this ~ ; structure, without the domains, was originally proposed for a-C2Sby Bredig (B7). Structure determinations have also been reported for u',-C,S (U8,13,12) and P-C,S (M9,J3).a',-C,S appears to have a structure similar to that of f3-K,SO, (B7). The a',--C,S structure is a more complex variant on that of a',, in which the 0.69 nm axis is tripled. The structures of the a',, a', and 0 polymorphs are derived from that of uC2S by progressive decreases in symmetry, which arise from changcs in the orientations of the Si0,'- tetrahedra and small movements of Ca'- ions. The quantities of impurity ions needed to stablize the higher temperature polymorphs at room temperature decrease along the sequence Srom u- to PC,S. In all these respects. the situation parallels that existing with the C,S polymorphs, described in Section 1.2.1. Also as in the latter case. and for the same reason, these C,S polymorphs differ in Ca2' ion coordination. In (3C,S, some of the C a Z f ions have 7, and others 8, oxygen atoms within 0.288 nm (52). The crystal structure of y-C,S is similar to that of olivine, (Mg,Fe),SiO, (Fig. 1.4D) (U4). The calcium is octahedrally coordinated. The unit cell and the arrangement of C a 2 +and SiO,,- ions show some similarities to those of
18
Cement Chemistry
-I
a = 0.558nm -I
I..
Fig. 1.4 Crystal structures of C,S polymorphs, after Udagawa ct (11. (U2. US). Large. open circles represent calcium atoms. small full circles. silicon atoms and triangles, tetrahcdra of oxygen atoms. Hcights of atoms are givcn as hundredths of thc cell height (0.68-0.71 nm). For a-C2S, the structure shown is that of an individual domain of space group P31c. The structures are shown in the relative orientations found when they are intcrconverted. an unconventional choice of origin being adopted for yC,S to clarify the relation to the other polymorphs.
Portland Cement and its Malor Constituent Phases
19
the other polymorphs, but also important differences. It has generally been supposed that transformations involving this form entail total reorganization of the structure, but single crystal high-temperature X R D evidence indicates that this is not correct; the products are formed topotactically (i.e. in a definite crystallographic orientation relative to the starting material: Fig. 1.4), but fragment because of the large volume change (U5.B8.G6). y-C2S scarcely reacts with water at ordinary temperatures.
1.3.2 Lamellar textures in clinker belites
The belite grains in Portland cement clinkers frequently show complex. striated structures. These have been studied over a long period by workers using light microscopy; Yamaguchi and Takagi ( Y I ) and O n o c / (11. (01). who also used X R D and other methods, gave the first substantially completc interpretations. Groves (G7) and Fukuda and Maki ( F I ) extended these results, using single crystal electron diffraction and other methods. A very common type of belite grain in production clinkers, called Type I belite, is rounded, typically 20-40 pm in mean dimension, and shows two or more sets of parallel striations. Within each set of striations. the laniellae extinguish simultaneously when examined in transmitted light between crossed polars and are therefore not twin lamellae; their origin is discussed in the next section. The lamellae in different sets d o not extinguish simultaneously. Within each set, secondary striations, not parallel to the primary ones, have sometimes been observed using light n~icroscopy,and are niorc clearly seen using electron microscopy. The secondary lamellae within a given set d o not extinguish simultaneously, but alternately at different angles, in the manner characteristic of polysynthetic twinning. Type I belite grains are those that have crystallized from the liquid at temperatures above about 1420•‹Cand thus as a-C2S.The primary striations arise on cooling through the u to u', transition, in which the symmetry decreases from hexagonal to orthorhombic, each set of striations thus representing a different orientation of the a', structure. The transition to a',C2S that occurs on further cooling does not further increase the number of orientations, but that from u', to p, in which the symmetry falls to monoclinic, causes each orientation to split into two. This is the cause of the secondary sets of striations described above. Belite crystals have been observed that show only one set of striations. whose behaviour in transmitted light between crossed polars shows them to be twin lamellae. Such crystals are typically irregular in shape; they have been called Type 11 belites, and are rare in modern clhkers. The striations arise from the a', to 3! transformation. Such crystals can form either by
20
Cement Chemistry
crystallization from the liquid below the u to u',, transition tempcruture. or by slow cooling of a-C,S under conditions such that one of the twin components of the resulting a' modification has grown at the expense of the others. These two modes of formation can be distinguished morphologically. the second mode giving larger and less rounded grains ( 0 1 ) . Belite forms in clinkers by additional mechanisms, sometimes at lower tenipcrat~~rcs than those discussed above, and may not then show any striations.
1.3.3
Polymorphic types of belites in clinkers
X R D powder evidence shows that in the majority of clinkers the belitc is predominantly or entirely of 0-C,S structure (G1.Y I ) , though some peaks are broadened ( G I ) and the presence also of both u and u' (presumably u',-) forms has been reported ( G I , Y I,RI,OI). Characterization of the polymorphic form is rendered difficult by the similarities between their powder patterns (Fig. 1.5) and by overlaps between the peaks and ones of other phases, especially alite, but has been aided by examination of fractions in which the belite has been concentrated by chemical (R 1 ) o r heavy liquid ( Y I ) separation.
30•‹
3S0
30'
DEGREES 20
35O (
30'
35"
CU Kal)
Fig. 1.5 Portions of X R D powder patterns (microdensitometer traces of Guinier photographs) of (A) a-C,S at 1 500"C, (B) a',-C,S at 1000•‹Cand (C) P-C,S. Indices are based on axes used in Table 1.4. After Regourd and Guinier (RI).
The striations in the common, Type I grains of belite are zones into which impurity ions rejected during the transition from a to a' have been concentrated (YI,Ol,Cl). In some cases, they appear to have retained the a-C,S structure (Y I,Ol,FI). In contrast, a study on synthetic belites showed that the material exsolved between the lamellae or a t grain boundaries was often
Portland Cement and its Malor Constituent Phases
21
compositionally far removed from C2S, and structurally either amorphous or composed of crystalline phases of entirely different structure, such as C,AS. C,A or C,S, (CI). Chan ct ul. ( C l ) concluded from this study. in which SEM and TEM with diffraction and X-ray microanalysis were used. that fl-C2S cannot be stabilized by excess CaO alone, as some previous workers had supposed. They also concluded that the possibilities of compositional variation in P-C,S are considerably narrower than has often been supposed. the grosser deviations from ideal composition that have been reported being attributable to exsolution of foreign ions in lamellae or at grain boundaries. They further concluded that the role of ionic substitution in stabilizing forms other than y to room temperature needed to be reassessed. mechanical constraints associated with exsolution efyects providing a possible, alternative explanation. Studies on synthetic belites suggest the possible occurrence of modulated structures, i.e. ones in which compositional or structural variations occur with a statistical periodicity that is not a rational multiple of that of the basic structure (H5). Such effects might occur in clinker belites.
1.3.4 Compositions of belites in clinkers
Table 1.2 includes an average composition for belites in ordinary production clinkers, based on the results of many studies by X-ray microanalysis (T2,K I .G3.B2,G4,SI .H3,H4) or chemical analysis of separated material (YI). As with alite (Section 1.2.3), most of the results from different laboratories are in relatively close agreement. Belites in white cement clinkers have also been analysed (B3). Pure C2S, for comparison, contains 34.9% of SiO, and 65.1 % of CaO. Clinker belites typically contain 4-6% of impurity oxides, of which the chief ones are usually A120, and Fe,O,. Correlations have been reported to exist between the MgO contents of belite and clinker (KI), the Fe,O, contents of belite and clinker (KI), and the total impurity content of the belite and the MgO content of the clinker (B4). Early reports mention a compound KC,,S,,, but the K 2 0 content of 3.5% corresponding to this formula is well above those found in clinker belites, and recent electron optical work indicates that the limit of K 2 0 substitution is about 1.2% (CI). In general, clinker belite compositions indicate ratios of other atoms to oxygen above the theoretical value of 0.75. As Regourd et UI.(R3) noted, the C2S structures are too densely packed for the presence of interstitial ions to appear likely, and i t is more probable that vacancies occur in the oxygen sites, with possible concomitant shifts in the positions of other atoms. Table 1.3 includes atomic ratios, with a possible allocation of atoms to sites.
22
Cement Chemistry
corresponding to the typical composition given in Table 1.2. Any interpretation of belite analyses is. however. at present tentative because of the uncertainties arising from exsolution effects. From the practical standpoint. i t may be necessary to regard the exsolved material as forming part of the belite.
1.3.5
Cell parameters, X-ray powder patterns and other data
(11. (R3) synthesized several belites of P-C2S type with compoRegourd sitions similar to those found in clinkers and determined their cell parameters. With increased substitution, the crystallinity tended to decrease. causing broadening of XRD powder peaks. Two of their results are given in Table 1.5. Table 1.5
compositions of synthetic belites ( R 3 )
Coniposition
n (nm)
h (nm)
c(nm)
Fig. 1.5 compares parts of the XRD powder patterns of the a. u', and P polymorphs. The appendix includes a calculated pattern for a typical clinker belite. Assuming u = 0.550 nm, h = 0.675 nm, (. = 0.933 nm, P = 94.4 , with the atomic parameters of Jost ct (11. (52) and site occupancies 0 . 0 , 0 . 0 0 s 0 . 0 . 0 l 2 ~ 0 . 0 " . 0 6 ~ 0 . 0 0 . 0 1. 0 1 3 . the X-ray density is 3300 kg m-" That for pure P-CIS, based on Jost ct cil.'s data. is 3326 kgm-3, and that for pure y-C2S, based on those of Udagnwa rt (11. (U4), is 2960 kg n ~ - ~ . Synthetic P-C,S is biaxial positive with high 2V and a = 1.7 17. y = 1.735; y-C,S is biaxial negative with 2V = 60•‹, a = 1.642, P = 1.645, y = 1.654 (R4). In the Type I belites of clinkers, lamellae of P structure have mean refractive index 1.720 and birefringence 0.015-0.01 8, while for the intervening material the corresponding values are 1.700-1.710 and 0.000-0.003 (01). Ono et uI. (01) and Yamaguchi and Takagi (Y 1) give further data on the optical properties of clinker belites. Guinier and Regourd (GI) summarized the thermal behaviour of C2S polymorphs. On heating, P-C,S shows endotherms beginning at 693•‹Cand 1450•‹C.due respectively to the P -+ a', and a', -, a transitions; with y-C,S, the 693•‹Cendotherm is replaced by a broad one beginning at about 748•‹C due to the transition to a',. The curve obtained on cooling a-C,S from
Portland Cement and its Major Constituent Phases
23
1500jC shows exotherms beginning at about 1456•‹C682'C and 530•‹C.due respectively to the u u',,, a', -+ P and P -+ y transitions. The a', -+ a', transition gives only a very weak efTect at I l6O"C. P-C2S, modified by sold solution, occurs in nature as larnite. Early identifications of u'-C2S with bredigite were incorrect (B9,M 10). u-C,S has been assumed to resemble nugclschmidtite (Ca,Si,P,O,,). hut the ~lnitcell of the latter seems to be more complex (GI). -+
1.4 T h e a l u m i n a t e p h a s e 1.4.1
Crystal structure: cubic, orthorhombic and monoclinic modifications
Pure C,A does not exhibit polymorphism. It is cubic, with a = 1.5263 nm, space group Pa3 and Z = 24; the structure is built from CaZ ions and rings of six A10, tetrahedra, of formula A1,O:;- (M 11). These rings are highly puckered, such that the aluminium atoms lie near to six of the corners of a cube (Fig. 1.6). The unit cell is composed of 64 (4,) subcells, of edge length 0.3816 nm. Of these subcells, 8 are occupied by A1,0,, rings; the C:I' ions +
+
n
Fig. 1.6 Al,O,, ring in the structure of C,A, showing the situations of the aluminium atoms near to the corners of a cube; based on the results of Mondal and Jeffery (MI 1).
24
Cement Chemistry
occur at the body centres of some of the remaining subcells and near to the corners of others. The coordination of those near the corners is octahedral. while that of those at the body centres is irregular, with either 5 or 6 oxygen atoms within 0.28 nm. C,A can incorporate Na" by substitution of C a Z + with inclusion of a second Na" ion in an otherwise vacant site, thus giving solid solutions of general formula Na,,Ca3 _,Al,O, (F2,R5,R I ,T4). The substitution occurs without change in structure up to a limit of about 1% Na,O (.Y r 0.04). Higher degrees of substitution lead to a series of variants of the structure (Table 1.6). In the absence of other substituents, the upper limit of NazO substitution is 5.7'/0. Tablc 1.6
Modifications ofthc C,A structurc. ofgcncrnl formula Na,,Ca,.vAI,O,
(T4) Approxi~natc Compositional rangc .Y Na,O (I%))
0 I .O 1 .O 2.4
2.4-3.7 3.74.6 3.6-5.7
0--0.04 0.04-0.10 0.10-0.16 0.16-0.20 0.20-0.25
Designation CI CII C,, + 0
0
M
Crystal systcln
Spacc group
Cubic Cubic
Pa 3 P2,3
-
Orthorhombic Monoclinic
Pbca P2, /a
The C, structure (Table 1.6) is the C,A structure, with up to about 1% of Na,O present as described above. The C,, structure is a minor variant of it. of lower symmetry (T4); comparison of calculated X R D patterns (T5) sho\\.s that there is little possibility of distinguishing the two by powder XRD. unless by very precise determinations of the cell parameter, C,,, with 2.4%) N a z O having u = 1.5248 nm (T4). In C,,, and by analogy also in C,, the additional Na' ions are located at the centres of the A1,0,, rings. The 0 structure (originally called 0 , )resembles C , and C,, in having a unit cell that is composed of pseudocubic subcells with edge lengths of approximately 0.38 nm. which contain C a 2 + ions and AI,O,, rings, but the arrangement of the rings within the true unit cell is entirely different (T4,N4). A preparation of composition Na,,,75Ca,,375A15,,75Fe Si,,,7,01,(3.3% N a 2 0 ) had u = 1.0879nm, h = 1.0845nm, c = 1.5106nm, Z = 4 (T4). The M structure, which was originally considered to be orthorhombic and called 0,, (R5). is a slightly distorted variant of the 0 structure; a preparation of composition Nal,,,Ca,~2,Al,0,, (5.7% Na,O) had LI = 1.0877 nm, h = 1.0854 nm, c = 1.5135 nm, = 90.1•‹,Z = 4 (T4). Substantial proportions of the aluminium in these structures can be replaced by other ions, of which Fe3 and Si4+ are the most important. Lee et LII.(LI) found the limits of substitution under equilibrium conditions to be
,,,,,
+
Portland Cement and its Major Constituent Phases
25
around 2% for SiOz and 3 ~4'21for Fe,O,, but higher degrees of substitution were obtainable under non-equilibrium conditions. such as crystallization from under cooled melts. If Si4+ is present. fewer Inrgc cations are nccded to maintain charge balance; this extends the solid solution range of the 0 structure to lower NazO contents and that of the M structure to highcr ones (T4). The literature contains ninny references to a supposed compound NC,A,. This formula corresponds to a Na,O content ol' 7.6'2). which cannot be attained in C,A substituted only with N a + . and i t is rcnsonnbly certain that this compound does not exist (F2,R5.M 12.T4); howevcr. ifSi4+ is also prescnt, the upper limit of Na' substitution is near to this value (M 12.T4). Pollitt and Brown (P2) were unable to prepare an analogue of the orthorhombic phase with K as the sole substituent, but obtained evidence that K + could stabilize i t in clinker, probably because other substituents were also present. Maki ( M 12) also failed to prepare K forms of the orthorhombic or monoclinic phases under equilibrium conditions, but by moderately rapid cooling of melts he obtained orthorhombic crystals having cell parameters close to those of the corresponding sodium-containing phase. He considered that the presence of silicon in the clinker liquid would favour supercooling and thereby also non-equilibrium formation of the orthorhombic or monoclinic phase. +
1.4.2 Other modifications Two further modifications of the C,A structure have been described. One was obtained as a high-temperature polymorph of the 0 and M forms and was considered on the basis of powder X R D evidence to be tetragonal (RI,R5). It appeared to be metastable at room temperature but could be preserved by quenching. Later studies. using thermal analysis, high-temperature light microscopy and high-temperature single crystal X R D (M 13,M 14,T4), showed, however, that the 0 modification was structurally unchanged up to its decomposition temperature and that the M modification was reversibly transformed on heating into the 0 form. A study of quenched material using powder X R D and single crystal electron diffraction (L2) confirmed that the supposed tetragonal form was really orthorhombic, though with u equal to h and thus geometrically tetragonal. The space group was considered to be probably Pcaa, which differs from that (Pbca) of the normal 0 form; however, three of the indices assigned to X R D powder peaks (023,045 and 047) are incompatible with this space group, though not with Pbca. The high-temperature material may thus have the normal 0 structure and a composition such that the a and h axial lengths are equal to
26
Cement Chemistry
within experimental error. Its persistence to room temperature appears to be favoured by relatively high contents of Na,O and Fe,O, (LI). The other C,A modification is a disordered, poorly crystalline form, called 'proto-C,A1, which was obtained nietastably from simulated clinker liquids, either by rapid cooling or by static crystallization at low temperatures (BIO). I t gave an X R D powder pattern with broadened peaks. corresponding to the systematically strong reflections of cubic C,A indexable on the subcell with u -0.39 nni. Analytical electron microscopy of individual crystals showed i t to bc very high in substituents, one preparation, for example, having an Fe/AI ratio of 0.54 (H6).
1.4.3 Structural modifications of the aluminate phase in clinkers
Production clinkers have been found to contain cubic or orthorhombic forms of the aluminate phase, alone or in combination. The monoclinic modification has not been observed. The orthorhombic modification is also known as the prismatic, dark interstitial material, and is sometimes pseudotetragonal. It can arise only if sufficient alkali is available, but its formation appears to be favoured also by rapid cooling and by bulk con-ipositions potentially able to yield a relatively high proportion of aluminate phase ( M 12). Cubic aluminate phase in clinker is often finely grained and closely admixed with dendritic crystals of ferrite; when it forms larger crystals. these tend to be equidimensional. The X R D powder pattern is characterized by strong, singlet peaks at approximately 33.3". 47.7" and 59.4 20 (CuK,, radiation; Fig. 1.7A), the indices of which are, respectively, 044. 008 and 448. Patterns obtained either from clinker or from material in which the aluminate and ferrite phases have been concentrated by chemical extraction of the silicates give u = 1.5223-1.5255 nm ( R I ) . The slight decrease relative to the value of 1.5263 nm for pure C,A agrees with the results for synthetic sodium-substituted preparations (RI,T4). These data are probably e q ~ ~ a l l y compatible with the presence of proto-C,A, but the contents of substituents (Section 1.4.4) are considerably lower than those found in the latter material. There is probably not enough evidence to show whether the modification is C , (Pa3) or C,, (P2,3). The orthorhombic phase is recognizable in the light microscope or SEM by its occurrence as characteristic, lath-shaped crystals, which are often twinned ( M 12). An X R D powder pattern of material from which the silicate phases had been removed showed a splitting of the strong peak at 33.3" 20 peak into a strong singlet at approximately 33.2" and a weaker, close doublet at 32.9"-33.0" (Fig. 1.7B). The unit cell parameters were u = 1.0874 nm, h =
Portland Cement and its Major Constituent Phases
27
1.0860 nm, c = 1.5 120 nm ( R 1). Another clinker, in which the aluminate phase was pseudotetragonal, gave a strong singlet peak at 33.2" and a weaker singlet at 33.0" (Fig. 1.7C), and the cell parameters were u = h = 1.0867nm. c = 1.5123 nm. The aluminate phase in clinkers can also be characterized by its composition, determined by X-ray microanalysis; this is discussed in the next section.
I
3 3 O
I
I
I
34O
33O DEGREES 2 8
L
I
1
3k0
33O
I
I
34O
(Cu K a l )
Fig. 1.7 Portions of X R D powder patterns of clinkers containing ( A ) cubic, ( B ) orthorhombic and (C) pscudotctragonal modifications o f the aluminate phase. Peaks marked A and F are due to aluminate and ferrite phases, respectively, and are reindexed, where necessary, to correspond to axes in the text and Table 1.7. and to calculated intensities. After Regourd and Guinier ( R I ) .
1.4.4 Compositions of t h e aluminate phase in clinkers Because of the close admixture with other phases, which is often on a scale of 10 pm or less, X-ray microanalysis of the aluminate phase in clinkers is frequently difficult or unreliable. Data have been reported for cubic, orthorhombic, pseudotetragonal or unspecified forms of the aluminate phases in ordinary clinkers (R1,K 1 ,B2.H3) and for aluminate phase (G3,G4,SI ,B3) and glass (B3) in white cement clinkers. Tables 1.2 and 1.3 include. respectively, average compositions based on these somewhat scanty data, and suggested site occupancies based on them. The values in both tables take into account both the experimental data and the requirement of reasonable site occupancies.
28
Cement Chemistry
Pure C,A contains 62.3% CaO and 37.7% A1,0,. Substantial proportions of both calcium and aluminium arc thus replaced. the total content of impurity oxides being typically around 13% for the cubic and up to about 20% for the orthorhombic modification. The content of equivalent Na,O (Na,O + 0.66K20)appears to be around 1% for the cubic form a n d 2 4'%1 for the orthorhombic form. None of the analyses indicates nlk ;I I'I contents as high as that required by the formula NC,A,, even though considerable amounts of silicon are present.
1.4.5 X-ray powder data, densities and optical properties The appendix includes calculated XRD powder patterns for cubic and orthorhombic clinker aluminates (T3). The X-ray density of tlie cubic modilicution is 3064 kg 111-", assuming rr = 1.5240 nm and composition -Ca2,76Mg,,,8Ti,,~,,)(Fe,,,,,AI Si )Oh.That of the ortho(K,,,Na rhombic modification is 3052 kg ni-" :nssuming rr = 1.0879 nm. 11 = 1.0845 nm. c = 1.5106 nni and composition ( N ~ I ~ , ~ ~(FcO. ~ C5O ~ I ~ , ~ ~ ~ ~ ) AI,,,,,Si )0,. For pure C,A with cr = 1.5263nm. the valuc is 3029 kg ni- ,. Pure C,A is optically isotropic, with refractive index 1.7 10 (R4); iron substitution can raise the value to 1.735 (H7). Maki (M 13.M 15) described the nlorphology and optical properties of the orthorhombic modification. including variations with composition.
,,,,
,,,, ,,,,,
,,,,,
,
1.5 The ferrite phase
1.5.1 Crystal structure and composition in the Ca,(AI,Fe,
.),O, series
At ordinary pressures in the absence of oxide components other than C a 0 , A1,0, and Fe,O,, the ferrite phase can be prepared with any composition in the solid solution series Ca,(AI,Fe, -,),O,, where 0 < r <0.7. Thc composition C,AF is only a point in this series, with s = 0.5. The end membcr C,A, with .r = I , has been prepared, but only at a pressure of 2500 Ml'n (Al). The series with .r < 0.7 is not quite isostructural, as the space group changes near s = 0.33 (S4). Table 1.7 gives crystal data for various conipositions. Care is needed in referring to the unit cell and space group. as some workers reverse the choice of a and c axes used here, with appropriate changes in space group symbol. There has also been uncertainty in tlie past as to the space group for the compositions with s > 0.33.
Portland Cement and its Major Constituent Phases
29
Table 1.7 Crystal data for fcrritc phases in the series Ca2(AI,Fcl. .,)?0, -
Unit ccll paralncters (nm)" Space group
X-ray density (kgm 7
RcScrenccs
Pcmn l bm2 lbm2 Ibm2 I bt112 ('?) -
"The unit ccll 1s orthorhomhic, with Z
=
4
Plots of unit cell parameters o r X R D powder spacings against .\- for the pure CaJA I,Fe),O, series show changes in slope near .\- = 0.3. attributable to the structural change (S4). Such plots may be used to determine composition provided that oxide components other than CaO, AI,O, and Fe,O, are absent. They have often been applied to ferrites in clinkers, but this gives seriously inaccurate results because of the etkcts of other substituents (Sections 1.5.2 and 1.5.3). Biissem (B12) determined the approximate crystal structure of C,AF. Subsequent determinations or refinements were reported for prcp;irations with .Y = 0 (C,F) (B13.C4,B1 I), s = 0.285 and 0.36 (C2) and .\- = 0.5 (C?). Fig. 1.8 shows the structure for thc compositional range with 0.33 < .Y < 0.7. It is built from layers of corner-sharing octahedra similar to those in perovskite (CaTiO,), alternating with layers composed ofchnins of tetrahedra. together with C a 2 + ions. The layers are perpendicular to the h axis and the chains are parallel to c. The composition of an individual octahedral layer in the ric cross-section of the unit cell is M 2 0 , , and th;it o f an individual chain is T,O,, where M and T denote octahedral and tetrahedral cations. respectively; two corners of each tetrahedron arc shared with adjacent octahedra. Including also the C a w ions, this gives the empirical formula Ca,MTO,. The structure for con~positionswith .Y < 0.33 differs from the above in that one half of the chains have the opposite polarity. Each C a 2 + ion in C,AF has 7 oxygen neighbours at 0.23-0.26 nm (C3). The aluminium and iron atoms are both distributed between octahedral and tetrahedral sites, the fraction of the aluminium entering tetrahedral sites under equilibrium conditions decreasing with temperature. For the three preparations with s = 0.285,0.36 and 0.5 on which X-ray structure dcterminations were made. 75-76% of the total content of aluminium was found to be in tetrahedral sites. These preparations were shown to have been in equilibrium at about 750•‹C ((23); for a C,AF preparation quenched from
30
Cement Chemistry
1290•‹C,the Mossbauer spectrum indicated that only 68% of the i~luminiuni was in tetrahedral sites (G8). It was suggested that .v does not cxcccd the observed limit of about 0.7 because at this composition thc tctrahcdral sites are all occupied by aluminium (C2). There is evidence of clustering of aluminium and iron atoms to an extent depending o n composition and conditions of formation (ZI).
Fig. 1.8 Crystal structure of C,AF, bascd on thc rcsults of Colville a n d Gellcr (C3) and showing calcium atoms (open circles). (AI.Fc)O, tctrahcdra (triangles) and (Fe.AI)O, octahedra (hatchcd squares). Heights of a t o m arc given as hundredths of the cell height ( h = 1.452 nm). Atoms at heights outsidc the rangc of - I to +39 arc not shown, their positions being derivable from those shown by translations of 113 parallel to each of thc axes.
1.5.2
Compositions of the ferrite phase in clinkers
In many clinkers, the ferrite phase is closely mixed with aluminate: due to a similarity in cell parameters, oriented intergrowth can occur ( M 15). The close admixture often renders X-ray microanalysis difficult or unreliable. For ordinary Portland cement clinkers, the compositions found in difl'erent laboratories are nevertheless remarkably consistent. Table 1.2 includes an average value based on the results of investigations using X-ray microanalysis (H8,K 1,B2,U I,H3,B4) or chemical analysis of separated material ( Y I ) . Table 1.3 includes suggested site occupancies corresponding to these data.
Portland Cement and its Major Constituent Phases
31
As with the aluminate phase, the average compositions take into account the requirement that these site occupancies should be reasonable from the gandpoint of crystal chemistry. There is no basis for allocating cations to octahedral and tetrahedral sites separately as the preferences of some of the cations, especially Mg2+,in this structure are unknown, as is the temperature at which equilibrium is attained. This temperature probably varies between clinkers, and may be expected to affect the distribution. The typical composition differs markedly from that of C4AF (46.1 CaO. 21.0% A1203, 32.9% Fe20,). I t contains about 10'%, of impurity oxides and is much lower in Fe20,. I t approxiniates to Ca2AIFeo,,Mgo,,Sio,,,Ti O,, which is derived from C,AF by replacing some of the Fe3+ by Mg2+ and an equal amount by Si4+ and Ti4+. Harrison c / (11. (H3) observed that, for data from several laboratories. the number of magnesium atoms per formula unit agreed with that of (Si + Ti); this supports the above interpretation. Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the con~positiongiven above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies.
,,,,
1.5.3 Crystal data and X-ray powder patterns for ferrite phase containing foreign ions
Mn3+ can replace all the Fe3+ or up to 60% of the A13+ in C4AF (K2). Yamaguchi and Takagi (Y I) summarized data showing that incorporation of Mg2+ or Si4+ or both causes increases in cell parameters. Boikova (B4) determined the cell parameters of four preparations with compositions similar to those of typical clinker ferrites; the ranges were u = 0.55350.5554 nm, h = 1.4492-1.4642 nm, c = 0.5288-0.5333 nm. Marinho and Glasser (M 17) found that Ti4+ substitution caused stacking changes in C 2 F or C4AF. I t had negligible effects on the cell parameters and the charge was balanced by incorporation of additional oxygen atoms. The last conclusion could be irrelevant to clinker ferrites, since Ti4+ is a relatively minor wbstituent and the charge could be balanced in other ways. Regourd and Guinier ( R I ) reported unit cell parameters for the ferrite phase in five clinkers. The ranges observed were u = 0.5517-0.5555 nm, h = 1.455-1.462 nm, c = 0.5335-0.5350 nm. Boikova (B4) reported X R D powder spacings for clinker ferrites which indicate similar values. The similarity of these cell parameters to those of the laboratory preparations
32
Cement Chemistry
(R4) mentioned in the previous paragraph supports the results of X-ray microanalyses of clinker fcrrites described in Section 1.5.3. Compared with pure C,AF. typical clinker ferrites have smaller values of rr and c, but larger values of h. The XRD powder patterns of clinker ferrites arc alkcted by the cooling rate ( M 18.02). Ono and Nagasmima ( 0 2 ) showed that the elTect was associated with differing uptake of MgO and S O z . Extreme v a l ~ ~of e sthe cell parameters were rr = 0.557 nm, 1) = 1.462 nm. c = 0.532 11111I'or a quenched sample. and rr = 0.543 nm, 11 = 1.465 nm, c = 0.533 n m Ihr one that was slowly cooled. The ferrite in the quenched samples was poorlj. cryst(I 11.~ n e , many peaks other than the three most intense (200, 141 and 302) disappcaring. Broadening of peaks of clinker ferrites (M 18.R I ,B4) might be caused not only by poor crystallinity, but also by zoning. Thc ccll pnromcters observed by Regourd and Guinier ( R I ) and Boikova (B4) arc near to those of the quenched sample. The appendix includes a calculated powder pattern li?r a ferrite phase having cell parameters and con~positionsimilar to those for the material in typical clinkers. The X-ray density, assuming rr = 0.5535 nm. 1) = 1.4642 nm, c = 0.5328 nni and con~position CazAIFe,,,,Mg,l,2Si ,,,,5Ti,,,,, is 3570 kg m--'. 1.5.4 Optical, magnetic and other data
Ferrite phase of or near C4AF composition and free from foreign ions is yellowish brown in transmitted light. The optical properties of C4AF are as follows: biaxial, negative, with moderate 2V: a 1.96, 2.01. y 2.04 in lithium light; pleochroic. with y brown, a yellowish brown; the refractive indices increase with Fe/AI ratio (H7). C,AF-MgO solid solutions, in contrast, are grey or black, and also have higher electrical conductivity, both effects being attributable to the presence of electronic defects (M18). Normally cooled clinkers are grey or black, but quenching in water or cooling in nitrogen causes them to be yellow; this has been attributed to absence of oxygen during cooling (M 18). The ferrite phase is ferromagnetic, and the more ironrich members of the Ca,(AI,Fe, -,),O, series have been studied for their magnetic properties, especially in regard to the coordinations of the iron atoms (C4,G9). Ferrite phase, with a composition near to C,AF, has been found in nature. The name brownmillerite, already in use in cement chemistry, was adopted as the mineral name ((35).
High-temperature chemistry 2.1
Introduction
A knowledge of the relevant high-temperature phase equilibria is necessary for understanding the factors that govern acceptable bulk compositions for Portland cement clinker, the conditions under which the latter can be manufactured. and the phase composition and microstructure of the resulting material. This chapter deals with these equilibria and with the phases to which they relate, with the exception of the major clinker phases, whicti wkre described in Chapter I. Some anhydrous phases primarily of interest in relation to other types of cement are also considered here. Principles underlying the preparation of anhydrous silicate, aluminate and other hightemperature phases are outlined.
2.2 Systems containing CaO w i t h SiO, or AI,O, or both
2.2.1 The CaO-SiO, system
Fig. 2.1 shows the phase diagram. For clarity, the polymorphism of C,S and the distinction between a',- and a',-C,S are omitted. Calcium oxide (CaO) has the sodium chloride structure, in which all ions are octahedrally coordinated; the unit cell is cubic, with u = 0.48105 nm, space group Fm3m, Z = 4, D, = 3345 kgm-3 (S5). The refractive index is 1.837 (W3). On equilibrium cooling below 1250•‹C,C3S decomposes to give CaO and a',-C2S; this process, which is always slow and below 700•‹Cimperceptible, is considered in Section 3.5.5. P-CS, a-CS and C,S2 are commonly known by their mineral names of wollastonite, pseudowollastonite and rankinite, respectively; none reacts significantly with water at ordinary temperatures. The crystal structures of P-CS (04) and a-CS (Y3) differ markedly from each other and the two polymorphs are easily distinguishable by powder XRD. Both exhibit polytypism ($-CS (H9,HlO); a-CS (Y3)); the polytypes of a given polymorph are barely distinguishable by this method. The name 'parawollastonite' has been used for the 2M polytype of P-CS. The p-a transition is reversible but slow in the a to P direction, rendering a-CS easily preservable by quenching. The structure of rankinite is known (S6); a
34
Cement Chemistry
polymorph, kilchoanite, is known only as a natural mineral and as a product of hydrothermal reactions. The polymorphs of silica relevant to cement chemistry are briefly considered in Section 3.3.2.
Fig. 2.1 The system CaO-SiO,. After Day et al. (Dl) with later modifications (R4, 0 3 . W2, G10).
2.2.2
The CaO-AI,O,
system
There have been many phase and equilibrium studies on this system, and differences of opinion probably still exist on several points. In part, these differences arise because variations in the humidity and oxygen content of the furnace atmosphere markedly affect the phase relations for compositions in
High-temperature Chemistry
35
the region of C,,A,. The phase diagram in Fig. 2.2 appears to provide the best description of the system modified by the presence of small amounts of water and oxygen, and thus represents the behaviour in air of ordinary humidity. With both CA and CA,, opinions have differed as to whether melting is congruent or incongruent; Nurse er a[. (N6), who found it to be incongruent in both cases, discussed earlier work. Of the five calcium aluminate phases appearing in Fig. 2.2, one, tricalcium aluminate (C,A), was described in Section 1.4.
Fig. 2.2 The system CaO-A120, modified by the presence of small amounts of H 2 0 and 02,and thus representing the behaviour in air of ordinary humidity. After Rankin and Wright (R4).with later modifications (N5,N6.C5).
Monocalcium aluminate (CA), which is the main constituent of calcium aluminate cements (Section 10.1), reacts rapidly with water. It is monoclinic and pseudohexagonal, with a = 0.8700 nm, h = 0.8092 nm, c = 1.5191 nm, p = 90.3", space group P2,/n, Z = 12, D, = 2945 kg m-,, and has a stuffed tridymite structure, composed of C a 2 + ions and an infinite, three-dimensional framework of A10, tetrahedra sharing corners (HI 1). It crystallizes as irregular grains, sometimes prismatic and often twinned, with a = 1.643, p = 1.655, y = 1.663, (-)2V = 36" (R4). Formation from CaCO, and AI(NO,), in the presence of organic reducing agents is reported to proceed through an amorphous material at 500•‹Cand an orthorhombic polymorph at 850•‹C(B14). Calcium dialuminate (CA,) occurs in some calcium aluminate cements. It reacts only slowly with water. The formula was at one time wrongly considered to be C3A,. CA, is monoclinic, with a = 1.2840nm, b =
36
Cement Chemistry
0.8862 nm, c = 0.5431 nm, P = 106.Z0, space group C2/c, Z = 4, D, = 2920 kgm-3, and has a structure based on a framework of A10, tetrahedra, in which some oxygen atoms are shared between two and others between three tetrahedra (B15,Gll). It crystallizes as laths or rounded grains, with a = 1.6178, P = 1.6184, y = 1.6516, (+)2V = 12" (B15). CA, has been found as a natural mineral (G5). Calcium hexaluminate (CA,) melts incongruently and does not react with water at ordinary temperatures. Its crystal structure is closely related to those of corundum and P-A1203. 2.2.3
C,,A, and derived structures
In air of ordinary humidities, a phase of approximate composition C12A7is readily formed. It reacts rapidly with water and occurs in some calcium aluminate cements. A related phase, CllA7CaF,, is similarly reactive and occurs in certain special cements. In early work, C12A7 was wrongly assigned the formula C,A3 and called 'stable C5A,' to distinguish it from another phase of that composition, described later, which was called 'unstable C5A,'. Cl,A, is cubic, with a = 1.1983 nm (J3), space group 143d and Z = 2 (B12). Studies on C12A7 or closely related phases (B16,W4,B17,W5,F3) show the crystal structure to be built from Ca2+ ions, an incomplete framework of corner-sharing A10, tetrahedra and empirical composition Al70::-, and one 02- ion per formula unit distributed statistically between 12 sites. At temperatures above about 930•‹C, a reversible equilibrium exists between C,,A7 and water vapour in the furnace atmosphere (N5,R6,J3,N7). Material prepared in ordinary air and quenched from 1360-1 390•‹Cis almost anhydrous; on being gradually reheated, it takes up water until at 950•‹Ca maximum content of about 1.3% is attained, corresponding to the formula C12A7H.On further increase in temperature, the water is lost again, until by the melting point in the region of 1400•‹Cthe material is almost anhydrous. The sorbed water affects the cell parameter and refractive index. There was early disagreement as to the direction of the variations, but it appears established that C12A7Hhas the smaller cell parameter and higher refractive index, Jeevaratnam et af. (53) finding a = 1.1983 nm, n = 1.611 and D, = 2680kgm-3 for C12A,, and a = 1.1976nm, n = 1.620 and D,= 2716 k g m - 3 for C12A7H. Jeevaratnam et a/. (53) suggested that C12A7can take up water because the 12 sites per formula unit that are occupied statistically by one 02-ion can alternatively be occupied by two O H - ions, C12A7Hthus having the constitution Ca6A17016(OH).In support of this conclusion, they prepared the halide analogues C, , A 7 C a F 2and Cl1A,CaCI2 of the latter compound.
High-temperature Chemistry
37
C , ,A7.CaF2was found to have a cell parameter of 1.1964 nm, giving D, = 2732 kgm-,. It forms a continuous range of solid solutions with Cl,A7 (S7). Bromide, iodide, and sulphide analogues (TI,Z2) and a chloride analogue with partial replacement of aluminium by silicon, balanced by additional occupancy by chloride of the statistically available sites (F3), have also been described. Imlach et al. (14) found that they could prepare C12A7reproducibly by slowly cooling melts in dry oxygen and that samples obtained in this way, or by cooling melts in moist air, contained excess oxygen, which could be detected chemically and was possibly present as peroxide ion. If similar melts were cooled in very dry nitrogen, mixtures consisting of CA, C,A and, sometimes, C,A,, were obtained, with only poor yields of C12A7.These observations have since been confirmed and extended (B18,B19,Z2,Z3). Zhmoidin and Chatterjee (Z2,Z3) concluded from density, viscosity and mass spectrometric evidence that melts of Cl,A7 composition contained regions of two kinds, differing in structure and density. The proportion of the less dense regions increased if the melt took up H 2 0 , O,, CO, CO, or SO,, or if CaF,, CaCI,, CaS, CaSO,. or CaCO, was added; if this proportion was sufficiently high, Cl,A7 or a derivative was formed on freezing. The proportion of the denser regions in the melt was increased by maintaining a dry and slightly reducing atmosphere and by increase in temperature; if the less dense regions were thus eliminated. the melt yielded a homogeneous glass on quenching, or C,A, on slow cooling. The molecules that stabilized the less dense regions acted by surrounding themselves with a particular type of open structure, from which C12A7 readily nucleated; analogies were noted with the roles of hydrated alkali cations in the formation of zeolites and of water or organic molecules in that of compounds of the noble gases. With the exception of water, the molecules thus occluded in crystalline Cl,A7 could not be removed without destroying the structure. C12A7melts are considerably denser (2870-2910 kg m-,) than solid C12A7,especially if they have been kept in reducing atmospheres. The derivatives of C12A7melt in normal ways, C, , A 7 C a F 2congruently at 1577•‹Cand C l , A 7 C a S incongruently to give CaS and liquid at 1482•‹C (22). In contrast, C12A7stabilized only by water shows anomalous behaviour, one study (N7) showing a sharp melting point of 1392•‹Cover a range of compositions, and another (Z2,S7) showing melting over a range from 1380 to 1415•‹Cwith evolution of gas and incipient formation of CA, C,A and C,A,. The diagram in Fig. 2.2 is thus approximate in this region. For practical purposes, the situation in air of ordinary humidities is more important than that in very dry and oxygen-free atmospheres, and the primary phase field of C12A7will be included in the relevant ternary and quaternary phase diagrams presented later.
38
Cement Chemistry
C12A7 or C12A7H has been found as a natural mineral and named mayenite (H 12,GS).
2.2.4 C,A3, C2A and C4A3 The conditions of formation of C,A3 were discussed in the preceding section; it is probably an equilibrium phase in the strictly binary system CaO-AI2O3 but does not form in atmospheres of normal humidity and oxygen content. C5A3is orthorhombic, with a = 1.1253 nm, h = 1.0966 nm, c = 1.0290 nm, space group Cmc2,, Z = 4, D, = 3067 kg m-3, and a structure related to that of gehlenite (Vl). The density is considerably higher than that of C12A7and accords with formation of the compound from the denser regions present in the melt. C5A3 reacts rapidly with water (B20). Two other anhydrous calcium aluminates are known. C2A, a highpressure phase, was described in Section 1.5.1. C4A3 is formed as the dehydration product of a hydrothermally produced phase, C4A3H3. Its structure, which is similar to that of sodalite (Na4(A13Si30,,)CI), is based on a framework of corner-sharing A10, tetrahedra with C a 2 + and 02-ions in interstices, giving the constitutional formula Ca4(AI,012)0 (P3).
2.2.5 The Ca0-A1203-Si0, system The phase diagram was originally determined by Rankin and Wright (R4). Fig. 2.3 is based on that of Muan and Osborn (M19), with further amendments mainly following from studies on the bounding CaO-SiO, and CaO-A1203 systems discussed in the preceding sections. It relates to atmospheric pressure in an atmosphere of normal humidity, the primary phase field of C12A7thus being shown. Some phases probably form solid solutions within the system, C3S, for example, being able to accommodate some A1203; this is not shown. Two ternary compounds exist stably in the system under these conditions. Gehlenite (C2AS; Ca2AI2SiO7)belongs to the melilite family. In its structure, layers of 8 coordinated Ca2+ ions alternate with ones of composition A12SiO;-, in which both aluminium and silicon are tetrahedrally coordinated (L3). Gehlenite forms extensive solid solutions, e.g. with ikermanite (C2MS2; Ca2MgSi207), but not within the Ca0-A1,03-Si0, system. It possibly forms as an intermediate compound in the manufacture of Portland cement clinker, but does not occur in the final product; it is present in some calcium aluminate cements. It is tetragonal, with a = 0.7716 nm, c = 0.5089 nm, space-group ~ 4 2 , r n ,Z = 2, D, = 3006 kg m - 3 (L3) and refrac-
High-temperature Chemistry
39
tive indices w = 1.669, E = 1.658 (W3). It does not react with water at ordinary temperatures. Anorthite (a polymorph of CAS,; CaAI2Si2O,), which is less relevant to cement chemistry, is a triclinic felspar.
Fig. 2.3 The system Ca0-A1,0,-Si0,. modifications.
After Muan and Osborn (M19), with later
Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na,KAI,Si,O,,) or kalsilite (KAISiO,) (Y4), are of possible interest in relation to the formation of calcium aluminate cements.
2.2.6 Clinker formation in the Ca0-A120,-Si0, The Ca0-AI20,-Si0,
system
diagram provides a basis for a preliminary under-
40
Cement Chemistry
standing of the chemistry underlying the formation of Portland cement clinker, in which all but the three most important oxide components are ignored. The approximations are least for white cements. In a cement kiln, the maximum temperature reached by the mix, called the clinkering temperature, is commonly 1400-1 450•‹C;at this temperature, the mix is partly molten. For a mix in the pure Ca0-A1,03-Si0, system, a somewhat higher temperature is required to give a comparable situation. In this discussion, we shall assume that equilibrium is attained at a clinkering temperature of 1500•‹Cand use the Ca0-A1,03-Si0, diagram to predict which solid phases will be present for various bulk chemical compositions. The processes by which the equilibrium is approached in cement making, and those taking place during the subsequent cooling, are discussed in Chapter 3.
Fig. 2.4 Part of the system CaO-A120,-SiO,, illustrating the formation of Portland cement clinker; for significance of lettered points and of thin full o r broken lines, see text.
The compositions in the pure Ca0-A1,03-Si0, system that correspond most nearly to those of typical Portland cement clinkers lie within the ringed area surrounding point P in Fig. 2.4. This point lies within the triangle whose apices are the compositions of C3S, C2S, and point X, which lies at the intersection of the boundary of the C3S and C,S primary phase fields with the 1500•‹Cisotherm. For any bulk composition in this triangle, the equilibrium phases at 1500•‹Cwill be C,S, C,S and liquid of composition X. This may be understood by considering what happens if a melt of composition P is cooled. C,S first crystallizes, the liquid composition thus moving away from that of C3S until the boundary between the primary phase fields of C3S and C2S is reached. C3S and C,S then both crystallize, and the liquid
High- temperature Chemistry
41
composition moves along this boundary until the 1500•‹C isotherm is reached. For the conditions that exist in cement making, the entire mix is never molten, so that the path to equilibrium differs, but the resulting phase assemblage must be the same. The crystals of C3S and C,S, being in contact with the melt, can grow to relatively large dimensions, which are typically around 30 pm. During the subsequent cooling, the liquid solidifies, producing the more finely grained interstitial material. Some other bulk compositions may be considered in the same way. A composition such as Q, lying within the triangle C,S-X-Y, will at equilibrium at 1500•‹Cgive C2S and a liquid having a composition on the curve XY. One such as R, in the triangle C3S-X-Z, will give C3S and a liquid having a composition on XZ. Any composition lying below the line C3S-Z, such as S, will give an assemblage that includes calcium oxide as a solid phase. In making Portland cement, it is essential to avoid the presence of more than minimal amounts of free calcium oxide in the final product, and ~ ~ o r m a ldesirable ly to maximize that of C3S. If equilibrium was continuously maintained during cooling, apart from decomposition of C3S into C,S and calcium oxide at sub-solidus temperatures, any bulk composition lying within the triangle C3S-C,S-C3A would yield a final product consisting of these three phases. For bulk compositions below the line C3S-Z, this would require that the crystals of calcium oxide present at the clinkering temperature be redissolved. This process is slow, and one cannot assume that it will be substantially completed in the conditions existing in the kiln. The line C3S-Z therefore sets a practical limit to the CaO content of the mix. For the temperature range in which clinkering is practicable, it virtually coincides with a line joining the C3S composition to the invariant point at 1470•‹C involving calcium oxide, C,S, C3A and liquid, and that line may therefore be used to define the upper limit of acceptable CaO contents. This approach, modified as discussed in Section 2.3.3 to allow for the presence of Fe203, leads to the definition of a quantity called the lime saturation factor, which can be used in practice as an important parameter of the bulk chemical composition.
2.3 Systems containing Fe,O, 2.3.1
The CaO-A1203-Fe203 system
This system includes the Ca2(A1,Fe, -,),O, series of ferrite compositions. The bounding, binary system CaO-Fe,O, includes three compounds, viz. C,F, C F and CF,. C,F, as an end member of the above series, was discussed in Section 1.5. The other two compounds are of lesser importance to cement
High-temperature Chemistry
51
and to C,A, and gehlenite (C2AS); this is shown by unit cell determinations (M23) and, for Phase Q, by a structure determination (H14), which also yielded a probable structure for C3A2M. In all the known structures mentioned above, aluminium, magnesium and, where present, silicon are tetrahedrally coordinated. Phase Q was originally assigned the formula C,A4MS (PS), and it has also been suggested that two distinct quaternary phases exist, with compositions C2,A14M7Sand C2,A,,M3S, (G13). The structure determination (H14) and an associated phase study (K3) indicated that there is only one such phase, and that its composition is Ca20A132-2,MgnSin0,8 with 11 variable between 2.5 and 3.5, a median formula thus being C20A13M3S3. The unit cell, for n = 3.0, was found to be orthorhombic with a = 2.7638 nm, h = 1.0799 nm, c = 0.5123 nm, space group Pmmn, Z = 1 and D, = 2985 kg m-,. A phase known as pleochroite, of fibrous or acicular morphology. occurs in some calcium aluminate cements. It appears to be Phase Q modified by partial replacement of Ca2+ by Fe2 and of A13+ by Fe3 (P5,G 13). X-ray microanalyses by Conjeaud (C6) appear compatible with the approximate which is a substituted composition Ca,oA122,,Fe~~~~g3,2Fe~,~Si335068 form of that given by the structure determination and having n = 3.5. The results of another EPMA determination (M24) are of uncertain significance because they show improbably large deviations from charge balance. Several phase equilibrium studies have been made on this part of the Ca0-Mg0-A1203-Si0, system (P5,B9,G13,K3). Phase Q has a primary phase volume in the system, but there are some uncertainties regarding the stable solid-phase assemblages that include it. C3A2M has a primary phase field in the Ca0-Mg0-A120, and Ca0-Mg0-A1203-Si0, systems (W7,B9). C7A5M is a metastable phase, and the formula given may only be approximate. +
+
2.5 Systems containing alkalis or SO, or both 2.5.1
Phases
Portland cement clinkers contain small amounts of alkalis and sulphates derived from the raw materials and fuel. Both alkalis and SO, can be present in the major clinker phases, but tend to combine preferentially with each other to form alkali or potassium calcium sulphates, and it is necessary to consider these components together. In addition, silicate and aluminate phases containing sulphate can form either as intermediates or in undesirable deposits in cement making, and a calcium aluminate sulphate is a major constituent of some expansive and other special cements.
42
Cement Chemistrv
chemistry. Phillips and Muan (P4), in a study of the binary system, found that C F melted incongruently at 1216•‹Cto give C,F and liquid, and that CF, decomposed a t 1155•‹Cto give C F and hematite (a-Fe203).In this and other systems containing Fe203, iron-rich mixes tend to lose oxygen when heated in air above 1200-1300•‹C, with consequent replacement of hematite by magnetite (Fe304). Fig. 2.5 shows part of the ternary system CaO-A1203-Fe203. C3A, C,,A, and CA can all accommodate some Fe3+;for C3A under equilibrium conditions at 1325"C, the limit is about 4.5%, expressed as F e 2 0 3 (M20). The ferrite phase in equilibrium with iron-substituted C3A can have compositions with .Y between 0.48 and 0.7 in the formula Ca,(AI,Fe,-,),OS; if CaO is also present, x is fixed at 0.48, i.e. the composition is close to C4AF. Some reduction of Fe3+ to Fez+ occurs when the ferrite phase is prepared from mixes with compositions in the Ca,(AI,Fe, -,),OS series in air; it leads to the formation of minor amounts of other phases, which are not observed when similar experiments are carried out in oxygen (M20).
CaO
WEIGHT 'lo
CZF
Part of the system CaO-Al,O,-Fe,O,. Hatched lines indicate solid solutions. Ferrite phase compositions are indicated by the quantity x, which relates to the formula Ca,(Al,Fe, - ,),O,. After Newkirk and Thwaite (N8) and Majumdar (M20). Fig. 2.5
High-temperature Chemistry
43
Swayze (S8) noted that equilibrium was often difficult to achieve in this system. One effect was the tendency for crystals of the ferrite phase to be zoned. For bulk compositions in the Ca2(A1,Fel -,),O, series, the liquid is of higher AI/Fe ratio than the ferrite phase with which it is in equilibrium; the crystals that are initially deposited on cooling such a liquid therefore have a lower AI/Fe ratio than the bulk composition of the mix. On further cooling, the AI/Fe ratio of the material deposited progressively increases. Equilibrium within the ferrite crystals is difficult to attain, causing them to remain zoned, with cores richer in Fe3+ and outer regions richer in A13+ than the mean composition. A second effect was the tendency for protected phases to be formed. If a liquid having a composition somewhat on the CaO-rich side of the boundary between the CaO and C3A primary phase fields (Fig. 2.5) is cooled calcium oxide is initially deposited and the liquid composition moves away from CaO and towards that boundary. When the latter is reached, and assuming that equilibrium were to be maintained, calcium oxide would redissolve, C3A would be deposited, and the liquid composition would move along the boundary. In reality, the C3A quickly surrounds the particles of calcium oxide, which thus form a protected phase, effectively removed from the system. This can markedly affect the composition of the ferrite phase which is subsequently formed on further cooling.
2.3.2
The Ca0-A120,-Fe,O,-Si0,
system
This system comprises the four major oxide components of Portland cement. Lea and Parker (L4,L5) made a classic study of the subsystem CaO-C,SC12A7-C4AF, which is the part directly relevant to cement production. Following Lea (L6), the formula Cl,A, is substituted in the present description of this and related investigations for C,A3, which was used by the original authors in accordance with contemporary opinion on the composition of the phase now regarded as C12A7.Any errors arising from this compositional difference will be small. Lea and Parker assumed the ferrite phase to have the fixed composition C4AF. Lea and Parker began by studying the bounding ternary subsystems CaOC,S-C4AF and C,,A,-C,S-C4AF. The first of these, which is the more important, is shown in Fig. 2.6, modified in accordance with later work on the CaO-SiO, system. In general form, it resembles the lime-rich corner of the Ca0-Al2O3-Si0, system (Fig. 2.4), with C4AF in place of C3A, the primary phase field of C3S thus being an elongated area extending away from the CaO-C,S edge of the diagram. The quaternary system may be represented on a tetrahedral model, each face of which represents one of the bounding, ternary systems. Within the
44
Cement Chemistry
Fig. 2.6 The system CaO-C,S-C,AF. modifications.
After Lea and Parker (L4), with later
tetrahedron, primary phase fields and subsolidus compatibility assemblages are represented by volumes, corresponding to the respective areas on the triangular diagram of a ternary system; correspondingly, isotherms are represented by surfaces. Fig. 2.7 shows some important features of the CaOC2S-C12A7-C4AF subsystem, as found by Lea and Parker and modified to take account of later work (S8,N8,W2,GlO,M20,S9). No quaternary compounds exist in this subsystem. The tetrahedron can therefore be divided into three smaller ones representing subsolidus compatibility assemblages for CaO-C3S-C3A-C4AF, C3S-C3A-C2S-C,AF and C,,A,-C3A-C2S-C4AF. For clarity, these are not shown on Fig. 2.7. The most important feature of the subsystem is the primary phase volume of C3S, which is a thin sliver, roughly parallel to the C2S-C,,A7-C,AF face of the tetrahedron and sandwiched between the larger phase volumes of CaO and C2S. In accordance with the bounding subsystems (Figs 2.4 and 2.6), two of its edges lie on the CaO-C,S-C4AF and CaO-C,S-C,,A7 faces of the tetrahedron, but it cannot extend to the CaO-C4AF-Cl,A7 face. Table 2.1 gives details of the eight invariant points involving C3S in this subsystem. Subsequent studies on the CaO-A1203-Fe203 (N8,M20) and CaOA1203-Fe203-SiO, (S8,S9) systems have extended knowledge of the latter to a wider range of compositions, but have only slightly affected conclusions regarding the phase volume of C,S. All indicate that the phase volume of
High-temperature Chemistry
45
C3A is larger, and that of the ferrite phase smaller, than was shown in Lea and Parker's diagrams, which in this region partly rested on early results for the CaO-C,,A,-C4AF system. To simplify the diagram, the volumes of phases other than C3S are only indicated in general terms on Fig. 2.7.
Fig. 2.7 The pseudosystem CaO-C,S-C,2A,-C,AF, showing the primary phase volume of C,S. For details of invariant points PI-P8, see Table 2.1. After Lea and Parker (L4). with later modifications.
2.3.3 Clinker formation in the CaO-A1203-Fe,03-SiO,
system
Lea and Parker (L5) derived a formula giving the maximum acceptable content of CaO in a Portland cement clinker composition. If equilibrium was maintained during cooling, apart from decomposition of C3S into C2S and CaO at subsolidus temperatures, any bulk composition within the tetrahedron C,S-C2S-C3A-C4AF would yield a clinker consisting of those four phases. However, as in the Ca0-A1,03-Si0, system, i t cannot be assumed that any calcium oxide present at equilibrium at the clinkering temperature will subsequently be reabsorbed. Following reasoning similar to
Table 2.1
Point
Invariant points involving C3S in the Ca&A1203-Fe20,-SiO,
Solid phases in addition to C3S
Liquid composition (weight CaO
CaO
c2s
+ + + + + + + +
CaO C3A C2S C3A CaO ferrite C,S + ferrite CaO C3A + ferrite CaO + C3A + ferrite CaO C3A + ferrite C2S C3A ferrite C2S C3A ferrite C2S C3A + ferrite
+ +
"sin formula Ca,(Al,Fe, -,),O,. E = eutectic; P = peritectic.
+ MgO
+ MgO
AI2O3
Fe,03
system and in the same modified by 5% MgO
Oh)
SiO,
MgO
x in ferrite phasea
Type of pointb
Temp. ("C)
Ref.
High-temperature Chemistry
47
that in Section 2.2.6, calcium oxide will not be present at the clinkering temperature of approximately 1450•‹Cif the bulk composition lies on the lime-poor side of a plane defined by the compositions of C3S, C4AF and the invariant point P, (Fig. 2.7 and Table 2.1). The oxide ratios by weight for these three compositions are given in Table 2.2. One may then set up three simultaneous equations of the type
where chemical formulae represent weight percentages of oxides in the clinker, and solve them to obtain an expression for the content of CaO at any point in the plane, viz: CaO
=
sSiO,
This gives s = 2.80, y = 1.18, z
+ yAI20, + ;Fe,O,
(2.2)
= 0.65.
Table 2.2
Oxide ratios by weight for points on the surface bounding the region in which calcium oxide is an equilibrium phase at 1450•‹C Point on surface
SiO,/CaO
A1203/Ca0
Fe,03/Ca0
c3S
0.357 0 0. 126
0 0.456 0.549
0 0.714 0
C,A F p3
+
+
The ratio [Ca0/(2.80Si02 1.18AI2O3 0.65Fe20,)] is called the lime saturation factor (LSF). A mix having an LSF greater than unity will yield free calcium oxide at the clinkering temperature, and this phase is liable to persist in the final product, irrespective of the degree of mixing of the raw materials and the time during which the clinkering temperature is maintained. The calculation is approximate, because of the neglect of minor oxide components and of ionic substitutions in the solid phases and for other reasons. For values below unity, the LSF provides a measure of the extent to which the maximum attainable content of C,S is approached. Values of 0.92-0.98 are typical of modern clinkers. Lea and Parker considered that their calculations were valid only for AI,O,/Fe,O, weight ratios in the clinker above that in C4AF (0.64), but this does not appear to be correct (W6). The plane within the CaO-C2S-C,,A,C4AF tetrahedron defined by the compositions of C3S, C4AF and the point P3 passes close to the composition of C,A, and therefore also to all other compositions in the Ca,(AI,Fe, -,),O, series. Swayze's (S8) results, discussed in the next section, show that the extension of the CaO-C3S
48
Cement Chemistry
boundary surface to more iron-rich compositions remains close to this plane, which therefore serves also for A120,/Fe203 ratios below that in C4AF.
2.4 Systems containing M g O or FeO
2.4.1 General
Portland cement raw materials contain small proportions of MgO; as noted in Section 1.1.2, these must be limited to avoid formation of more than a minor amount of periclase. The iron in Portland cement clinkers is normally present almost entirely as Fe3+. but calcium aluminate cements may contain both Fe3' and Fe2+. Table 2.3 lists some phases containing MgO that are in varying degrees relevant to cement chemistry. It is not a complete list of phases with essential MgO in the Ca0-Mg0-A1,0,-Si0, system. As seen in Chapter I , some MgO is also taken up by all four of the major clinker phases, typical contents being 0.5-2.0% for alite, 0.5% for belite, 1.4% for the aluminate phase, and 3.0% for the ferrite phase. Magnesium oxide (periclase), like calcium oxide, has the sodium chloride structure; it is cubic, with a = 0.4213 nm, space group Fm3m, Z = 4, D, = 3581 k g m - 3 (S5) and refractive index 1.7366 (W3). FeO (wiistite) has the same structure, but always contains some Fe3 +,balanced by vacancies. It forms extensive solid solutions with MgO and can also accommodate some CaO. Early studies of such systems as MgO-C,S-C,,A, (H13), CaO-MgOC,S-C,,A, (M22) and MgO-C4AF (15) indicated that periclase is the only phase with essential MgO liable to be formed in Portland cement clinkers. A later study on the Ca0-Mg0-A1,03-Si0, system suggested that bredigite and 'Phase Q' could possibly occur in rapidly cooled clinkers (B9), but there is no convincing evidence that either of these phases is present in production clinkers.
2.4.2
Effect of MgO on equilibria in the Ca0-A1203-Fe203-Si0, system
Swayze (S8,SlO) extended the work of Lea and Parker (L4,L5) by considering the effects of variable ferrite composition and of the presence of MgO as an oxide component. In the work with MgO, a constant 5% of the latter was present in all the compositions examined. This was considered sufficient to saturate the liquid, as small amounts of periclase were detected in nearly all the products. Fig. 2.8 shows the results for the system in the presence of
High-temperature Chemistry
49
Table 2.3 Some phases containing MgO relevant to cement chemistry
Name Periclase Forsterite Monticellite Merwinite Bredigite Akermanite Enstatite Diopside Spinel -
'Phase Q'
Formula
Structure type
Note
Sodium chloride
I
Olivine
C3MS2 C7MSA C~MS; MS CMS, MA GAsM(?)
I
Related to alkali subhate structures M h t e (C2AS, etc.)
1
Pyroxene Spinel
2
Related to C,AS and C,A,
3
C20A,3M3S3
Notes I. 'Phase T'. of approximate composition C, ,M, ,S (SI 1. G12. S12. S13) is identical with bredigite (B9. M21). 2. Also the polymorphs clinoenstatite and protoenstatite. which too are of pyroxene type. 3. For references and descriptions, see Section 2.4.3. Phase Q is of variable conlposition: the forniula given is a median.
50
Cement Chemistry
MgO, modified to take account of later work on the subsystems. The primary phase is thus always periclase, and the system is represented by using a tetrahedron whose edges correspond to a total weight percentage of 95%. Volumes within this tetrahedron relate to the second phase to crystallize. The most important effects, whether of taking into account variable ferrite composition or the presence of MgO, concern the invariant points for C3S, C,A, ferrite phase, liquid and either CaO or C,S, data for which are given in Table 2.1, where they may be compared with those of Lea and Parker. In the absence of MgO, the temperatures of the corresponding points agree well between the two studies, but the liquid compositions are slightly different. In the presence of MgO, the temperatures are each lowered by 37 deg C and the liquid compositions substantially affected, the A1,03 concentrations being increased and those of CaO and Fe203decreased. Swayze found the ferrite phase in equilibrium with C3A, C3S, C,S and liquid to have a composition slightly more iron-rich than C,AF if MgO was absent, and one close to C,A,F if MgO was present. Swayze pointed out that these results had important consequences for the composition of the ferrite phase in Portland cement clinkers, the compositions of which normally include significant amounts of MgO. He concluded that, for A1203/Fe203ratios above 1.6, the ferrite phase composition would be C,A,F. Probably a majority of Portland cement clinkers have A1203/Fe203ratios in this range. For ratios below 0.84, assuming no free calcium oxide to be present, no C3A would be formed, and the A1203/Fe20, ratio in the ferrite phase would be that of the bulk composition. For intermediate values, lack of information about the tie lines for the ferrite phase made it impossible to predict the composition of the ferrite phase. Analyses of the ferrite phase in typical clinkers (Section 1.5.2) show that these conclusions are essentially correct, though the situation is complicated by the fact that the ferrite phase contains significant proportions of minor oxide components, especially MgO, SiO, and TiO,. Swayze assumed that, for the bulk compositions that he studied, all the MgO was in the liquid phase at equilibrium, apart from the small proportions present as periclase. As noted in the preceding section, all four of the major clinker phases take up significant proportions of MgO. The contents of the latter component in the liquid phase were therefore probably substantially lower than 5%.
2.4.3
Phases structurally related t o gehlenite
The phases C3A,M,. C,A,M and 'Q' are structurally related to each other
Table 2.4 Crystal data for sulphate phases
Formula
Name
Crystal systema
Arcanite Ca langbeinite Aphthitaliteb Thenardite Anhydrite 'Aluminosulphate' 'Silicosulphate'
Or Or Tr Or Or Cu Or
Unit cell parameters (nm)
"Or = orthorhombic. T r = trigonal (y = 120"). Cu = cubic. Data are for composition given; cell parameters decrease with increasing Na/K ratio (P2).
Space group Pnam P2,2,2,(?) ~ 3 1m Fddd Amma 14,32 Pcmn
Z 4 4 Z1
8 4 16 4
D,
(kgm-,)
Ref.
Table 2.5
Optical properties of sulphate phases Refractive indiccs
Formula
Name
a
b
Y
Character
2V
-
K2S04
Kc& K3NS4 Na2S04 CaSO, C4A33 C5S2S
Arcanite Ca langbeinite Aphthitalitea Thenardite Anhydrite 'Aluminosulphate' 'Silicosulphate'
"Data are for composition given; refractive indices decrease with increasing Na/K ratio (W3)
Biaxial + Biaxial Uniaxial + Biaxial Biaxial + Isotropic Biaxial -
+
67" Low -
84" 43" -
60"
Ref.
54
Cement Chemistry
The only phase containing essential alkali but not essential SO, known to occur in Portland cement clinkers in more than trace amounts is the orthorhombic aluminate phase described in Section 1.4. Trace amounts of alkali carbonates (P2) or potassium aluminate (F4) have been reported to occur in some clinkers, and some other alkali phases are formed as intermediates or deposits. Tables 2.4 and 2.5 list the sulphate phases and give crystal data and optical properties, respectively. Arcanite is the polymorph (p) of KS stable at room temperature; it transforms reversibly to a-KS at 583•‹Cand melts at 1069•‹C(R7). Thenardite is the polymorph (V) of NS stable in the presence of moisture at room temperature; on heating, it passes through a complex sequence of phase transitions and melts at 883•‹C (El). Other polymorphs can persist to room temperature. Anhydrite is the polymorph (p) of CS stable at room temperature, formed on heating the hydrates or the metastable ~ C ('soluble S anhydrite'). A thermal effect at 1195"C, generally attributed to polymorphic change to an a-cS, may be associated with onset of ionic rotation (F5). On heating in air, decomposition is detectable below 1000-C (K5), but slow below 1200•‹C; in a closed container, anhydrite melts at 1462•‹C(R7). Calcium langbeinite undergoes a minor phase transition at 940•‹C and melts incongruently to give cs and liquid at 101 1•‹C (R7). Aphthitalite is essentially a solid solution phase of general composition (K,Na),SO,. When it is formed stably at room temperature, the composition does not vary much from K,NS,, but at higher temperatures compositions much higher in sodium are possible, up to a limit near KN,S,, and the resulting solid solutions can be quenched to room temperature. Glaserite is a varietal name for aphthitalite of or near K,NS, composition. c,A,S [Ca4(AI,0,2)(S04)] is readily formed on heating mixtures of appropriate composition in air at 1350•‹C(R8,H 15,H 16). The crystal structure is a slightly distorted variant on that of sodalite [Na,(AI,Si,O,,)CI]; it is composed of a three-dimensional framework of A10, tetrahedra sharing corners, with C a 2 + and SO:- ions in the cavities (H15). The distortion causes a doubling of the lattice parameter and change in space group (H16). c,A,S reacts readily with water, and melts a t about 1600•‹C(H 15). c,s,S [Ca,(SiO,),(SO,)] has sometimes been called sulphospurrite. The name is misleading, because the structure is unrelated to that of spurrite [Ca,(SiO,),(CO,)] and elemental prefixes conventionally denote similarity not only in formula type, but also in structure. c,s,S is isostructural with silicocarnotite, Ca,(PO,),(SiO,) (B21). It is obtained on heating mixtures of appropriate composition in air of ordinary humidity (G14,P7), and in this environment is stable up to 1298•‹C(P8). Pliego-Cuervo and Glasser (P7) were unable to prepare it in the absence of water vapour and obtained evidence that it contained a small amount of essential O H - . They con-
High-temperature Chemistry
55
sidered that the atmosphere in a cement kiln was sufficiently moist to stabilize it in the appropriate temperature range, but that this might not hold true for the centres of clinker lumps.
2.5.2
Equilibria
Considering first systems of sulphates alone, phase equilibria have been reported for KS-NS (El), KS-CS (R7,P9) and KS-NS-cs (B22). Melting of KS-NS-cs mixes begins below 800•‹C(R9). At high temperatures, KS is completely miscible with NS (El) and accommodates up to 24 mole % of CS (R7) in solid solutions having the a-KS structure. With increase in temperature above ambient, the compositional range of aphthitalite solid solution rapidly broadens, extending much further in the direction of NS and contracting slightly in that of KS; the maximum Na/K ratio of about 3 : 1 is attainable at 195•‹C(El). At room temperature, all the phases other than metastable aphthitalites have essentially the fixed compositions shown in Tables 2.4 and 2.5. A study of the C~O-KS-CS system showed that the sulphate melts dissolved only a little CaO (P9). Alkali sulphate melts also show only limited mutual miscibility with oxide melts similar in composition to the main clinker liquid (T6,P7,PIO,Tl). For contents of alkali sulphates of 2-3% in the clinker liquid as a whole, the sulphate liquid is dispersed in the oxide liquid as microregions about 100 nm in diameter (TI), but with higher contents of alkali sulphates, the two liquids separate on a macroscopic scale (TI,G15). The partitioning of the components between sulphate melts and simulated clinker liquids at 1350•‹Chas been studied (G15). SO, is virtually insoluble in the oxide liquid, and S O 2 , AI20, and F e 2 0 3 only dissolve to very minor extents in the sulphate liquid, but CaO, N a 2 0 and K 2 0 are appreciably soluble in both. For five mixes studied, the ratio of K 2 0 to N a 2 0 was between 1.9 and 4.6 times higher in the sulphate than in the oxide liquid. Several early phase equilibrium studies on systems of calcium silicates or aluminates with alkalis were reported (N9), but their significance needs to be reassessed because they postulated the existence of the compounds NC,A, or KC2,SI2, which in neither case is supported by more recent work. Gutt and Smith (G16) reviewed earlier work on the effects of SO, on the formation of Portland cement clinker, and to augment the available information studied part of the Ca0-A1203-Fe20,-SO2-CaS04 system at 1400•‹C.They concluded that, if A1203 was present and MgO absent, even small proportions of SO, could restrict or prevent formation of C,S. The effect was lessened, but not eliminated, if MgO was present, and it was noted
56
Cement Chemistry
that other minor components, especially alkalis, might further modify the situation. Pliego-Cuervo and Glasser (P8) determined subsolidus phase assemblages in the CaO-rich part of the Ca0-A1203-SO2-SO, system at 950-1 150•‹C. Four-phase and three-phase assemblages defining the equilibria between CaO, C,S, C,A, C,,A,, CA, c,s,S and c,A,S were established. This work CaOwas extended (P10) to cover parts of the c~o-AI~o,-s~o~-KS, and c~o-AI~o,-F~~o,-KS-CS ~1~0,-KS-cS, C~O-AI~O,-S~~~-KS-CS systems, and the role of sulphur-containing species in the solid, liquid and vapour states in the formation of Portland cement clinker was discussed. Neither C,S,S nor c,A,S could occur in the final product, but both could play important parts in the kiln reactions. Kapralik et al. (K5,K6) studied equilibria in part of the CaO-A1203s~o~-F~~o,-M~o-cS-KS system at temperatures up to 1300•‹Cwith reference to the formation of clinkers designed to contain C,A,S. Compatible phase assemblages were established. The authors noted that the subsystem C,S-c,A,S-CS included all the main compounds having hydraulic properties that were formed in clinkers of this type.
2.6 Systems with other components
2.6.1
Fluorides and fluorosilicates
Small amounts of F - may occur in Portland cement clinkers, arising from raw materials or deliberate additions to lower the clinkering temperature. Two ternary phases are known to exist in the CaO-C2S-CaF, system, both of which were discovered by Bereczky (B23). One, now known to be of formula 2C2SCaF2or Ca,(SiO,),F,, has a structure analogous to that of chondrodite [Mg,(SiO,),(OH,F),], with layers of p C 2 S structure alternating with ones containing Ca2+ and F - (G17). The second ternary phase has been described as 3C,SCaF2 (GIs), ', C,,S4CaF2 (T7) and Cl,S,.2CaF2 (G19), but its probable formula is ' Ca,-,,2Si201,-xFx (x I), which has a structure related to that of C,S with partial replacement of 02-by F - balanced by omission of Ca2+ (P1 l,P12). The arrangement of Ca2+, SiOt- and 0'- or F - ions differs from that in the C,S polymorphs, and is the simplest possible in the broader family of structures to which the latter belong (Pll). There is a strongly ' marked pseudocell, which (for x = 1.0) is hexagonal with a = 0.7099 nm, c = 0.5687 nm, space group P6,/mmc and atomic contents 112 [C,,S,CaF,], but the true cell is triclinic, with a = 2.3839 nm, b = 0.7105 nm, c = 1.6755 nm, a = 90.0•‹, = 1 17.3", y = 98.56" and atomic contents
-
57
High-temperature Chemistry
5[C,,S4CaF2] (PI 1,P12). The X-ray density, for s = l .O, is 2936 kg m-3. For brevity, we shall call this phase C,,S4CaF,. In the C,S-CaF, system, the two end members form a eutectic at 1 1 10•‹C. 2C2SCaF2can be obtained by solid-state reaction at 950•‹Cand decomposes at 1040•‹Cto give C,S and CaF, (G18). Two phase equilibrium studies of the CaO-C,S-CaF, system (T7,G20) (Fig. 2.9) are in good agreement. Cl,S4CaF2 melts incongruently at 1 170•‹Cto give C3S and liquid. In the presence of CaF,, C3S can thus be obtained at this temperature through the successive formation and decomposition at lower temperatures of 2C2SCaF2 and Cl,S4.CaF2 (B23,G21 ,T7).
.-
.
CaO
1360
ca 0
10
20
30
40 50 W E I G H T % CaF2
60
70
80
Fig. 2.9 The system CaO-C,S-CaF,. After Gutt and Osborne (G20; Building
Research Establishment; Crown Copyright). There is some uncertainty as to the extent to which F - enters the C,S structure (as opposed to that of C,,S,CaF,) and also about the effectiveness of C,S produced in presence of CaF, as a cement. Welch and Gutt (W8) found that addition of CaF, to the starting materials caused the polymorph to change from triclinic to monoclinic, and obtained results suggesting that 0.74% of F - entered the C,S; CaF, accelerated the formation of C,S but the compressive strengths of mortars made with the latter were reduced. Tanaka et al. (T7) found that C,S formed from Cl0S4.CaF2 was trigonal (presumably rhombohedral), though it tended to invert to monoclinic on quenching. They found that C,,S,CaF2 gave poor strengths and suggested that the poor hydraulic activity of Welch and Gutt's preparations might have been due to presence of this phase, formed from the C,S during cooling. Maki et a/. (M27) described an alite prepared from a melt containing F-; it was monoclinic (M,), and contained 0.9% F and 3.4% A1,03.
58
Cement Chemistry
In contrast to some of these results, Moir (M7) described a clinker containing 1.8% A1203, 2.3% SO, and 0.16% F, which gave high early strengths; the C,S it contained was rhombohedral. Shame and Glasser (S15) found that little F - could enter the C,S structure in the absence of A13+, which permitted coupled substitution to occur, giving material of general formula Ca,(AI,Si, -,.)(O,-,F,). The upper limit of x was 0.15 (1.2% F). At x = 0.15, the material was rhombohedral, even if cooled rapidly, and gave high 28-day strengths. In the presence of A13+ and F-, C,S could be made by solid-state reaction at temperatures as low as 1025"C, though reaction was slow below 1050•‹C.At higher temperatures, F - not lost by volatilization was mainly present in the melt, and thus acted as a flux rather than as a mineralizer. Another calcium silicate fluoride, cuspidine [Ca4(Si207)F2],exists but is not formed under conditions relevant to clinker formation. Systems containing fluorosilicate ions [SF;-] and the possible use of calcium fluorosilicate as a flux have been investigated (S16). In the CaO-A1203-CaF, system, the compound C l l A 7 C a F 2 was described in Section 2.2.3. One other ternary phase, of compositon 3CACaF,, is known; it is hexagonal, with a = 1.729 nm, c = 0.701 nm, and refractive indices o = 1.628, E = 1.618 (L7). Several studies on parts of the system have been reported (G22,C5,C7,Z4). 2.6.2 Carbonates Spurrite [Ca,(Si04),(C03)] is readily formed under suitably low partial pressures of CO,; using oxalic acid as a source of CO,, it has been obtained at 430•‹C (H17). At sufficiently low CO, pressures or sufficiently high temperatures, it decomposes to give C,S and CaO. Some other syntheses, and conditions under which spurrite is formed or decomposed in the cement kiln, are mentioned in Sections 3.3.3 and 3.3.5. A pressure-temperature equilibrium curve for the reaction of wollastonite and calcite to give spurrite and CO, has been reported (H18). The crystal structure is known, and the unit cell is monoclinic, with a = 1.049 nm, b = 0.6705 nm, c = 1.416 nm, = 101.3", space group P2,/a, Z = 4, D, = 3024 kg m - 3 (S17). The refractive indices are: a = 1.638, = 1.671, y = 1.676; (-)2V = 40" (H18). Another calcium silicate carbonate, tilleyite [Ca,(Si,O,)(CO,),], occurs as a natural mineral and has been synthesized (H17), but is not known to form during the manufacture of clinker. 2.7 Laboratory preparation of high-temperature phases In general, high-temperature phases are prepared in the laboratory by
High-temperature Chemistry
59
heating mixtures of appropriate composition at temperatures at which reference to the phase equilibria shows them to be stable. Platinum apparatus is normally required. Compounds having a lower temperature limit of stability (e.g. C3S) may have to be cooled rapidly to a temperature at which they are kinetically stable. Some high-temperature polymorphs can be reserved by quenching, but with others it may be necessary to add a stabilizer. For P-C,S, B,O, (0.1-0.3%) has often been used, but it may be considered preferable to employ a composition closer to that of the material in clinker. As noted in Section 1.3.1, crystals of P-C,S below a certain size do not invert to y-C,S on cooling. This makes it possible to prepare P-C,S without any stabilizer by first preparing y-C,S and reheating at 1000•‹C (S18). For the same reason, P-C,S is the normal product when hydrated calcium silicates of appropriate composition, including the major hydration product of C3S or Portland cement, are decomposed at 800-1000•‹C. 'As starting materials for high-temperature preparations, finely ground and well-mixed combinations of crystalline oxides (e.g. quartz) and carbonates (e.g. calcite) have been widely used. Mixtures containing carbonates must initially be heated at a relatively low temperature (900-1000•‹C for calcite) to avoid violent decarbonation. Repeated heating and regrinding, followed each time by examination of the product by light microscopy, XRD or other methods, is often necessary to produce an acceptable product. Intermediate products are often formed; for example, in the CaO-SiO, system, C,S is formed initially, and reacts only slowly with any excess of calcium oxide or silica to give other phases. Some phases can be obtained by cooling of melts or devitrification of glasses. Particular care and choice of conditions may be needed if, as with the ferrite phase, protected phases or zoned crystals may be formed. More reactive starting materials can sometimes be used; for example, C3S of 99% purity has been obtained by a single heating of a mixture of freshly precipitated calcium oxalate and hydrous silica (06). It may also be possible first to prepare a hydrated compound and then to heat it; for example, C3A can be obtained in this way via C,AH,. If material can be lost by volatilization, as with phases containing alkalis, fluoride, or sulphate, it may be necessary to use a sealed platinum container and to test its effectiveness by chemically analysing the product. Control of the furnace atmosphere, e.g. by employing mixtures of CO and CO, to buffer the oxygen pressure, may be needed if variable oxidation states are possible, as with iron compounds. Special methods may be needed to make single crystals of sufficient size for X-ray structure determinations or other purposes. These are described in papers on X-ray structure determination. Single crystals of C,S can be obtained from CaCI, melts (NIO).
The chemistry of Portland cement manufacture 3.1
3.1.I
General considerations
Summary of t h e reactions in clinker formation
In the manufacture of Portland cement clinker, the raw materials, typically a lin~estoneand a clay or shale, are intimately mixed and heated, ultimately to a temperature of about 1450•‹C.The principal reactions taking place are conveniently divided into three groups, viz: (I) Reactions below about 1300•‹C, of which the most important are (a) the decomposition of calcite (calcining), (b) the decomposition of clay minerals, and (c) reaction of calcite o r lime formed from it with quartz and clay mineral decomposition products to give belite, aluminate and ferrite. Liquid is formed only to a minor extent at this stage, but may have an important effect in promoting the reactions. At the end of this stage, the major phases present are belite, lime, aluminate and ferrite. The last two may not be identical with the corresponding phases in the final product. (2) Reactions at 1300-1450•‹C (clinkering). A melt is formed, mainly from the aluminate and ferrite, and by 1450•‹Csome 20-30% of the mix is liquid. Much of the belite and nearly all the lime react in the presence of the melt to give alite. The material nodulizes. to form the clinker. (3) Reactions during cooling. The liquid crystallizes, giving mainly aluminate and ferrite. Polymorphic transitions of the alite and belite occur. Fig. 3.1 shows these changes for a typical clinker. N o attempt has been made to show a detailed sequence of phases below 1 300•‹C,as sufficient data d o not exist, and minor phases, including sulphates, are omitted. Quantitative phase compositions at various stages vary considerably with starting materials and other factors.
3.1.2 Lime saturation factor, silica ratio a n d alumina ratio Chemical analyses of cements, clinkers and individual phases are commonly 60
The Chemistry of Portland Cement Manufacture
61
I-
I
E
CALCITE
w
3
> m
V)
Z
I-
-IRON I
400
OXIDES 1
I
I
800 TEMPERATURE
I
I
COOLING
1200 (OC)
Fig. 3.1 Schematic diagram showing the variations in typical contents of phases during the formation of Portland cement clinker. Loosely based on a figure by Wolter (W9).
expressed in terms of oxide components, but it is often useful to employ quantities derived from these. In the equations that follow, chemical formulae also denote weight percentages. The following parameters are widely used in the UK and elsewhere: Lime saturation factor (LSF) = Ca0/(2.8Si02
+ 1 .2A1,03 + 0.65Fe,03) (3.1)
Silica ratio (SR) = Si02/(A1203+ Fe203) Alumina ratio (AR) = A1,03/Fe20,
(3.2) (3.3)
The definition of LSF is theoretically based (Section 2.3.3), and in the form ' given above applies to clinkers; if corrected by subtracting 0.7S03 from CaO, it may be applied to cements. It largely governs the ratio of alite to belite and also shows whether the clinker is likely to contain an unacceptable proportion of free lime, a value of 1.0 or above indicating that the latter will be present at equilibrium at the clinkering temperature and thus liable to persist in the product. In practice, values up to 1.02 may be acceptable; typical values for modern clinkers are 0.92-0.98. Other parameters similar in
62
Cement Chemistry
function to LSF are used in some countries. A suggested modification to the LSF definition, intended to allow for magnesium substitution in alite, replaces CaO by (CaO + 0.75Mg0) for MgO < 2%, or by (CaO + 1.5) for MgO > 2% (S19). In the German literature, Kalkstandard I1 corresponds approximately to LSF and Kalkstandard I11 to this modification of it. The SR and AR, also respectively called silica modulus and alumina modulus, are empirically based. For normal types of Portland cement clinker, SR is usually 2.0-3.0, and AR 1.0-4.0, but these ranges do not apply to special types, such as sulphate-resisting or white cement clinkers. The SR governs the proportion of silicate phases in the clinker. Increase in SR lowers the proportion of liquid at any given temperature in the kiln, and thus makes the clinker more difficult to burn. The AR governs the ratio of aluminate to ferrite phases in the clinker, which has important efrects on cement properties, and also determines the quantity of liquid formed at relatively low temperatures; at 1338•‹C.for a given total content of A1,0, and Fe20,, the quantity of liquid theoretically passes through a maximum at an AR of 1.38. The proportions of raw materials are con~monlycalculated on the basis of the above parameters, most obviously by setting up and solving simultaneous equations; to fix n parameters, n + 1 raw materials of appropriate composition are required. It is also necessary to consider the desired or allowable contents of minor components.
3.1.3 The Bogue calculation In another approach, widely used in the USA and elsewhere, the quantitative phase composition is estimated using a procedure due to Bogue (B24). It is necessary also to know the content of free lime, which may be determined by a chemical extraction method (Section 4.3.3). The calculation is as follows: (1) Assume that the compositions of the four major phases are C,S, C,S, C3A and C,AF. (2) Assume that the Fe203 occurs as C4AF. (3) Assume that the remaining A120, occurs as C3A. (4) Deduct from the CaO content the amounts attributable to C4AF, C,A and free lime, and solve two simultaneous equations to obtain the contents of C3S and C,S. This leads to the following equations, in which CaO is assumed to have been corrected for free lime:
The Chemistry of Portland Cement Manufacture
63
As with LSF, the approach is applicable to cements if CaO is further corrected by deducting 0.7S03. Because minor oxide components are ignored, the total for the four main phases plus free lime will not add up to 100%. I t is implicit in the approach that all the MgO is assumed to occur as periclase. The phase composition calculated by Bogue's method is related to the LSF in that a ratio of C,S to C3A < 0.546, calculated without correcting the CaO content for free lime, corresponds to an LSF 3 1.0 (D2). The results of the Bogue calculation are often called potential phase compositions, because when the procedure was devised, it was generally considered that the principal source of error was failure to reach equilibrium during cooling. The results do indeed differ, probably often markedly, from the true phase compositions, notably in underestimating alite and overestimating belite (Section 4.4.6), and it is unlikely that equilibrium is maintained during cooling, but the direct source of error is that the compositions of the clinker phases differ considerably from those of the pure compounds. Bogue compositions are used in some specifications, and for proportioning by setting up and solving equations to calculate the relative amounts of raw materials needed to obtain given 'potential' contents of C3S or other phases. They have often been misused for other purposes on the assumption that they are close to the actual phase compositions. Spohn et 01. (S19) have indicated the dangers of such uncritical use. 3.1.4
Enthalpy changes in clinker formation
The enthalpy change on formation of Portland cement clinker cannot be calculated with high precision, mainly because of uncertainties associated with the clay minerals in the raw material. Table 3.1 gives data for the main thermochemical components of the reaction, almost all of which have been calculated from a self-consistent set of standard enthalpies of formation, and which are therefore likely to be more reliable than other values in the literature. The conversion of the clay minerals into oxides is an imaginary reaction, but valid as a component in a Hess's law calculation. Few reliable thermochemical data exist for clay minerals; those for pyrophyllite and kaolinite can probably be used with sufficient accuracy, on a weight basis,
64
Cement Chemistry
for other 2 : 1 and 1 : I clay minerals, respectively. The data for the pure compounds may similarly be used for the clinker phases. Table 3.2 illustrates the use of these data. The quantitative phase composition of the clinker was estimated from the bulk composition by the method described in Section 4.4. The overall enthalpy change is often calculated approximately from the bulk composition using a forn~ula(Z5), and an experimental method for its determination has been described (C8). Standard enthalpies of reaction
Table 3.1
Reaction
AH (kJ)
For I kg of
CaCO, (calcite) -+ CaO + CO,(g) AS4H (pyrophyllite) -,a-A1203 + 4Si0, (quartz) + H,O(g) AS,H, (kaolinite) a-Al,O, + 2Si0, (quartz) + 2H20(g) 2FeO.OH (goethite) -,a-Fe,O, + H,O(g) ZCaO + SiO, (quartz) -+ P-C,S 3Ca0 + SiO, (quartz) -+ C3S 3Ca0 + a-A1203 -+ C3A 6Ca0 + 2a-A1,03 + a-Fe,O, -+ C,A,F 4Ca0 + a-A1203 + a-Fe203-,C4AF
+ 1782 +224 +538 +254 -734 -495 -27 -157 - 105
CaCO, AS4H AS2H2 FeO.OH CIS C,S C3A C,A,F C,AF
-+
Values for starting materials and products at 25•‹C and 0. I01 MPa, calculated from the data of Wagman el ul. (WIO) excepting those for the formation of C,A,F (NI I ) and C,AF (TX).The value for C,AF is for 20•‹C.
Table 3.2 Enthalpy of formation of 1 kg of a Portland cement clinker
Starting materials treated as
Products treated a s
Calcite Quartz Pyrophyllite Kaolinite Goethite
c3s
1.20 kg 0.10 kg 0.15 kg 0.04 kg 0.03 kg
(3-(3
C3A C&2F CaO
Component of reaction
A H (kJ)
CaCO, CaO + CO, AS,H (pyrophyllite) -+ a-A1203 + 4Si0, (quartz) + H,O(g) AS2H2(kaolinite) -,a-A1203 + 2Si0, (quartz) + 2H20(g) 2FeO.OH (goethite) -+ a-Fe,O, + H,O(g) 3Ca0 + SiO, (quartz) C3S 2Ca0 + SiO, (quartz) -+ P-C,S 3Ca0 + a-A1203+ C3A a-Fe203 C,A,F 6Ca0 2a-A1,0, -+
-+
+
Total
+
0.673 kg 0.133 kg 0.1 18 kg 0.064 kg 0.010 kg
-+
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65
The overall enthalpy change in forming clinker is dominated by the strongly endothermic decomposition of calcite. The component reactions for the replacement of clay minerals by oxides are endothermic, because the heat required for dehydroxylation exceeds that liberated on forming the products. The formation of P-C2S from lime and quartz is moderately exotherniic, but that of C,S from lime and P-C,S is endothermic, with AH = +59 kJ kg-'. All these calculations refer to reactants and products at 25OC and 0.1 MPa. The enthalpy changes at the temperatures a t which the reactions occur are somewhat different, because the specific heats of reactants and products are not the same. The reaction of lime with C2S giving C3S changes from endothermic to exothermic at 1430•‹C (54). For the decomposition of calcite at 890% AH is + 1644 kJ kg- ' (L6).
3.2 Raw materials and manufacturing processes
3.2.1
Raw materials and fuel
These, and manufacturing processes, will be considered only to the extent needed for a basic understanding of their chemistry: fuller accounts are given elsewhere (D3,G23,K7,PI 3). The raw mix for making Portland cement clinker is generally obtained by blending a calcareous material, typically limestone, with a smaller amount of an argillaceous one, typically clay o r shale. It may be necessary to include minor proportions of one or more corrective constituents, such as iron ore. bauxite or sand, to increase the proportions of Fe203, A1,03 or SiO,. respectively. On the other hand, some argillaceous limestones and marls have compositions near to that required, making it possible to use a blend of closely similar strata from the same quarry. Limestones vary in physical characteristics from compact rocks of low porosity to friable and highly porous ones, such as chalk, which may contain LIPto 25% of water. All consist essentially of calcium carbonate, normally in the polymorphic form of calcite. Other naturally occurring forms of CaCO,, such as shell deposits, are sometimes used. Many limestones contain significant amounts of minor components, either as substituents in the calcite o r in accessory phases, some of which are deleterious if present in amounts exceeding a few per cent (e.g. MgO, SrO), a few tenths of a per cent (e.g. P,O,, CaF,, alkalis) or even less (some heavy metals). Suitable shales and clays typically have bulk compositions in the region of 55-60% SiO,, 15-25% A1,03 and 5-10% Fe203, with smaller amounts of MgO, alkalis, H 2 0 and other components. Mineralogically, their main constituents are clay minerals, finely divided quartz and, sometimes, iron
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oxides. The dominant clay minerals are usually of the illite and kaolinite families, but small amounts of smectites (montmorillonite-type minerals) may also be present. Kaolinite has a I : I layer silicate structure and ionic constitution A12(Si205)(OH),. Illites and smectites have 2 : 1 layer silicate structures, derived from those of pyrophyllite [A12(Si,05)2(OH)2]or talc [Mg,(Si205),(OH)2] by various ionic substitutions and incorporation of interlayer cations and, in smectites, water molecules. In place of clays or shales, other types of siliceous rocks, such as schists or volcanic rocks of suitable compositions, are sometimes used. Pulverized coal is the most usual fuel, though oil, natural gas and lignite are also used. The contributions of the fuel to the clinker composition must be taken into account, especially with coal or lignite, which produce significant quantities of ash broadly similar in composition to the argillaceous component. The fuels also contribute sulphur. Due to the increasing cost of energy, the need to preserve the environment, and the non-existence or exhaustion of suitable natural raw materials in some areas, industrial and other waste materials are of interest as possible raw materials or supplementary fuels or both. Energy can be saved if even a part of the CaO can be provided by a material, such as blastfurnace slag, that does not require decarbonation. Supplementary fuels include such materials as used or reject tyres and pulverized household refuse, which can be introduced into the system in various ways. Some materials, such as pulverized fuel ash (pfa; fly ash) can serve as raw materials that also possess some fuel content. Other wastes that have been used include calcium silicate residues from aluminium extraction, mining residues, and precipitated calcium carbonate from various industries.
3.2.2
Dry and wet processes; energy requirements
Comminution and mixing of the raw materials may be carried out either dry or wet. The theoretical amount of heat needed to produce 1 kg of clinker from typical raw materials is about 1750 kJ (Section 3.1.4). Additional heat is required because heat is retained in the clinker, kiln dust and exit gases after they leave the system, lost from the plant by radiation or convection, and, in the wet process, used to evaporate water. Table 3.3 compares the heat requirements of the wet process and a modern version of the dry process, in each case assuming typical raw materials and proper plant design and operating conditions. The greater thermal efficiency of the dry process occurs largely because there is no added water to be evaporated. The total energy requirement of a cement works includes also electrical energy used in operating the plant, to which grinding of raw material and of clinker make
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important contributions. This is higher for the dry process than for the wet process, typical values being I20 k w h and 77 kW per tonne of cement (432 kJ kg-' and 277 kJ kg-' respectively, including that used for grinding the clinker in each case (D4)) but the difference is small compared with that in the amounts of fuel required. The wet process has certain advantages if the raw materials are already soft and moist, but its low fuel efficiency has rendered it obsolescent, and the following description is of a modern version of the dry process. Table 3.3 -
Heat requirement in dry and wet process kilns (kJ/kg of clinker) Drv
Wet
Theoretical heat requirement for chemical reactions Evaporation of water Heat lost in exit gases and dust Heat lost in clinker Heat lost in air from cooler Heat lost by radiation and convection
1807 13 623 88 427 348
1741 2364 753 59 100 682
Total
3306
5699
Dry kiln with suspension preheater. Data adapted from Ziegler (26).
3.2.3 The dry process; suspension preheaters and precalciners
The raw materials first pass through a series of crushing, stockpiling, milling and blending stages, which yield an intimately mixed and dry raw meal, of which typically 85% passes through a 90-pm sieve. With automated, computer-controlled procedures, the LSF, SR and AR can be maintained constant to standard deviations of 1 %, 0.1 and 0.1, respectively. The raw meal passes through a preheater and frequently also a precalciner before entering a rotary kiln. A preheater is a heat exchanger, usually of a type called a suspension preheater in which the moving powder is dispersed in a stream of hot gas coming from the kiln. Fig. 3.2 shows a common arrangement, which employs a series of cyclones. Heat transfer takes place mainly in co-current; the raw material passes through the preheater in less than a minute, and leaves it at a temperature of about 800•‹C.These conditions are such that about 40% of the calcite is decarbonated. It is possible to introduce a proportion of the fuel into a preheater, with a corresponding reduction in the proportion fed to the kiln; a precalciner is a furnace chamber introduced into the preheater into which 50-65% of the total amount of fuel is introduced. often with hot air ducted from the
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cooler. The fuel in a precalciner is burned at a relatively low temperature; heat transfer to the raw meal, which is almost entirely convective. is very efficient. The material has a residence time in the hottest zone of a few seconds and its exit temperature is about 900•‹C; 90-95% of the calcite is decomposed. Ash from the fuel burned in the precalciner is effectively incorporated into the mix.
Fig. 3.2 Schematic diagram of a four-stage cyclone-type suspension preheater. P indicates the position at which a precalciner burner may be incorporated.
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Because less heat is supplied to the kiln, precalcination allows the rate at which material can be passed through a kiln of given size to be greatly increased, thus saving on capital cost. Alternatively, the rate of providing heat can be reduced, which lengthens the life of the refractory lining. Less NO, is formed, since much of the fuel is burnt at a low temperature, and with s o n x designs, NO, formed in the kiln may be reduced to N,. Low-grade fuels can often be used in the precalciner. Precalcination may also make it economically feasible to deal with the problem, discussed in the next section, of formation of deposits in the duct carrying the hot gases from the kiln to the preheater.
3.2.4 The rotary kiln
The rotary kiln is a tube, sloping a t 3 4 % from the horizontal and rotating at 1 4 rev min-', into which the material enters at the upper end and then slides, rolls o r flows counter to the hot gases produced by a flame at the lower or 'front' end. In a system employing a precalciner, the kiln is typically 50-100 m long and its ratio of length to diameter is 10-15. The maximum material temperature, of about 1450•‹C, is reached near to the front end of the kiln in the 'burning zone', also called the clinkering or sintering zone, in which the material spends 10-15 min. The kiln is lined with refractory bricks, of types that vary along its length in accordance with the varying gas and material temperatures. The bricks become coated with a layer of clinker. which plays an essential part in the insulation and in extending their life. Nodules of clinker, typically 3-20mm in diameter, are formed in a semisolid state in the burning zone, and solidify completely on cooling. which begins in a short cooling zone within the kiln, and continues in a cooler. In modern plants, when the nodules leave the kiln, their internal temperatures are around 1350•‹C, but their surface temperatures are considerably lower. Liquid or pulverized solid fuels are blown into the kiln through a nozzle with 'primary' air. Additional 'secondary' air is drawn into the kiln through the clinker cooler. The flame in the rotary kiln must meet several requirements. The clinker must be correctly burned, so as to minimize its content of free lime, with the least expenditure of fuel. The ash from a solid fuel must be uniformly absorbed by the clinker. For normal Portland cements, the conditions must be sufficiently oxidizing that the iron is present as Fe3+; however, for white cements, mildly reducing conditions may be preferable. Proper flame control also extends the life of the refractory lining of the kiln. Computer-aided o r fully automated control of kiln operating conditions is increasingly used.
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3.2.5 Circulation of volatiles; dust; cooling of clinker In the hotter parts of the kiln, the potassium, sodium, sulphur, chlorine and some other elements provided by the raw material o r fuel are partly or wholly volatilized. The fractions that are not pass into the clinker. The volatilized material is swept back with the hot gases to the cooler parts of the system, where, assuming that a suspension preheater or precalciner is used. i t is largely reabsorbed, so that cycles are set up. Material that is not reabsorbed may leave the system through the preheater as gas or dust. but some can form solid or semisolid deposits. Deposits formed in this and other ways can seriously restrict the movement of material and gases; in the kiln, they form rings. They can form in the duct carrying the hot gases from the kiln to the preheater. With some raw materials or fuels, it is necessary to divert part of the gas through a bypass and to filter out the dust that i t carries, and to accept the consequent losses of heat and material. but changes in the raw materials or fuel may sometimes be a preferable solution (D5). Dust from the kiln is largely captured in a suspension preheater; that which is not is removed by an electrostatic precipitator before the gas passes up the chimney. The flow of gas is aided by an exhaust fan. The dust is as far as possible returned to the system by mixing with the raw meal or with the fuel (insufflation), though the extent to which either is possible may be restricted by its content of alkali sulphates o r chlorides, which if high could lead to an unduly high recirculating load, conducive to the formation of deposits or undesirably high concentrations in the clinker. The clinker cooler is essentially a heat exchanger that extracts heat from the clinker for return to the system; also, a cooled clinker is more readily transported, ground or stored. Rapid cooling from the clinkering temperature down to 1100•‹Cproduces a better quality clinker (Section 3.5.3, and the clinker should be effectively air quenched as soon as it leaves the burning zone. When the clinker enters the grinding mill, its temperature should preferably be below 110•‹C.Current practice favours grate coolers, in which the clinker passes over moving grates through which air is blown. More air is needed for cooling than for combustion in the kiln, especially with a fuelefficient process. Some of the excess ('tertiary air') may be ducted to a precalciner or used to dry the coal and raw materials, but a quantity normally remains for which it has proved difficult to find economically viable uses, and which is therefore cleaned and exhausted; steam raising for electricity generation has been suggested as a possible use.
3.2.6 Other processes for clinker production; clinker grinding In the wet process, the raw material is fed as a slurry directly to a rotary kiln,
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which typically has a length to diameter ratio of about 30 and may be up to some 200 m long. In surviving wet process kilns, the slurry typically has a water content of 30-35%. A system of chains near the back end assists heat transfer. Cycling of volatiles and of dust is much less pronounced than with preheater systems. In the semi-wet process, some improvement in fuel efficiency is obtained by pressing out part of the water to produce a cake containing somewhat below 20% of water which can be fed to the kiln directly or through certain types of preheater. In the semi-dry or Lepol process, the raw materials are made into nodules with just over 10% water content which are fed to a moving grate preheater. The fuel efficiency is lower than that obtainable with a suspension preheater. Combined processes have been brought into operation in various countries for making cement and other materials, such as aluminium, iron or sulphuric acid ((324). While there are problems associated with such processes, the latter may become increasingly important as classical raw materials become scarcer and environmentally acceptable ways of disposing of industrial wastes more difficult. Portland cement clinker has a long storage life, and while it may be ground immediately, there are often good economic reasons for grinding it intermittently, and not necessarily in the same plant. To produce Portland cement, it is ground together with gypsum. Natural gypsum, which is generally used, commonly contains significant proportions of such impurities as anhydrite, quartz, calcite and clay minerals. In some countries, byproduct gypsums from various industrial processes are used. A proportion of anhydrite may also be added. Some national specifications permit the addition of materials other than calcium sulphate. To make interground composite cements, widely varying proportions of granulated blastfurnace slag, pfa or other materials are added (Chapter 9). Grinding typically requires a power consumption of 35-50 k w h tonne-' (125-180 kJ kg-'), almost all of which is converted into heat, and cooling is required. 3.3 Reactions below about 1300•‹C 3.3.1
Decomposition of carbonate minerals
The dissociation pressure of calcite reaches 0.101 kPa (1 atm) at 894•‹C(S20) and the decarbonation reaction is highly endothermic (Section 3.1.4). The rate of decarbonation becomes significant at 500-600•‹C if a sufficiently low partial pressure of CO, is maintained or if the calcite is intimately mixed with materials, such as quartz or clay mineral decomposition products, that react with the calcium oxide. Even in a precalciner, such mixing occurs, aided by agglomeration caused by the presence of low-temperature sulphate melts.
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In the absence of other substances, dolomite [CaMg(CO,)J begins to decompose rapidly in air at about 750•‹C, giving initially periclase and a carbonate of higher Ca/Mg ratio. The decomposition temperature is much affected by the presence of other substances. The mechanism and kinetics of calcite deconlposition have been much studied. The reaction proceeds by the movement inwards from the surface of an interface, behind which the material is converted into lime, thus producing a highly porous pseudomorph. The interface moves a t a constant rate, implying that the rate at any instant is proportional to the area of the interface. In principle, the rate is controlled by the slowest of the following five steps: ( I ) Transfer of heat to the exterior surface. (2) Transfer of heat from the exterior surface to the interface. (3) Chemical reaction a t the interface. (4) Transfer of CO, to the exterior surface. (5) Transfer of CO, away from the exterior surface. Hills (H19) showed that, contrary to some earlier opinions. i t was not possible to determine which is the rate-controlling step either from the observed relationship between the extent of reaction and the time, or from the apparent energy of activation, obtained by applying the Arrhenius equation to the rate constant determined at different temperatures. For large particles and low CO, pressures, the temperature within the sphere could be as much as 45 deg C below that of the surrounding gas, indicating that heat transfer controlled the rate. For the conditions existing in a rotary kiln, in which decomposition occurs in a deep, moving mass of material, heat transfer determines the rate (B25). A rotary kiln is well suited to the later stages of clinker production, but much less so for calcination. which if carried out in it requires some tens of minutes for completion. Very different conditions exist in a precalciner, where the raw meal is dispersed in hot gas. The reaction still proceeds by the movement of an interface inwards, but the rate is controlled by the chemical reaction, and the temperature within the particle is virtually that of the surrounding gas (V2,B26). The rate is much higher than in a rotary kiln, and. as seen in Section 3.2.3; decomposition is normally 90-95% complete within a few seconds.
3.3.2 Decomposition of clay minerals and formation of products Laboratory experiments in which single substances or mixtures are heated in stationary containers in air provide a starting point for understanding the
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that occur in manufacturing clinker, though caution is needed in applying the results because of the important differences in conditions. The behaviour of clay minerals on heating depends on their structure, composition, crystal size and degree of crystallinity. In general, any interlayer or adsorbed water is lost at 100-250•‹C; dehydroxylation begins at 300400•‹C and is rapid by 500-600•‹C. Clay minerals in which A13+ is the predominant octahedral cation do not form phases of radically different structure as the immediate products of dehydroxylation, but undergo varying degrees of structural modification and disorganization due to the replacement of 2 0 H - by 0 2 - . Kaolinite gives a poorly crystalline product, called metakaolin; with illites, the structural disorganization is less marked, perhaps because less water has to be lost. Above about 900•‹C,all begin to give new, crystalline phases, which are, typically, AI-Si spinel, cristobalite and, above 1 100•‹C,mullite. Quartz undergoes a minor, rapid and reversible phase transition to aquartz at 573•‹C.It is unstable relative to tridymite at 867-1470•‹C. and to cristobalite above 1470•‹C.These changes are slow in the absence of other materials, but many admixed materials accelerate them, and lead to formation not of tridymite, but of cristobalite at temperatures over about 1000•‹C. As noted in Section 3.3.1, the decarbonation of calcite is greatly favoured by intimate mixing with quartz or clay minerals. Under these conditions, much CO, is lost before any free lime can be detected. The formation of C,S as an early product is well established, but the situation with the calciunl aluminate phases is more complex, no one phase being dominant as an initial product (Wll). The aluminate or aluminosilicate phases most often reported as early or intermediate products in laboratory experiments with pure chemicals or raw meals have been CA, C,,A, and, less frequently, gehlenite (W 12,D6-D8,W 13,L8,L9,16,C9,CIO,R 10). C,A can form by 850•‹C(L9,CIO) but seems usually to be a later product. Ferrite phase forms readily, and is initially of low AI/Fe ratio (D7,RlO). The reactions are notably accelerated by water vapour (C10). In the cement kiln or precalciner, agglomeration may influence their course. Glasser (G25,G26) reviewed kinetic studies. The complexity of the mechanisms has not always been recognized, and it is doubtful whether kinetic data sufficeto determine them, but electron microscopy and X-ray microanalysis are beginning to provide reliable information (R1 1).
3.3.3 Sampling from cement kilns or preheater outlets
Several investigators have examined samples from rotary kilns during normal operation. In one such study, Weber (W13) obtained samples, and
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measured temperatures, mainly using sampling ports in the kiln walls. In all cases, the raw materials were limestone and marl, and the fuel was pulverized coal. CA and C12A7,but not gehlenite, were found as early products. As in the laboratory studies, much calcite decomposed before any free lime was detected. The free lime content passed through a maximum value of up to 17%, referred to the weight of clinker, about 15 m from the front end of the kiln. At this point, the measured temperature was about 1000•‹C. Laboratory studies (16) and analyses of kiln input meals from precalciners, described later, show that the maximum content of free lime reached varies greatly with the nature of the raw material. One may also examine samples from kilns that have been shut down, though uncertainties inevitably exist due to possible phase changes and movement of material during cooling, which is carried out with the kiln still rotating. Moore (M28) reported an XRD study of charge and coating samples taken from numerous points along the lengths of several kilns. Sulphate phases were prominent in both types of sample taken from the cooler parts of the kiln, and comprised anhydrite, C,A& C,S,S, calcium langbeinite and hydroxyl-ellestadite ( c 1 , s 3 S 3 ~ )KC1 . was sometimes found. Spurrite ( c , s , ~ ) , p-C,S, C12A7and ferrite were usually also found before all the calcite had decomposed. Gehlenite was usually found in coating samples at intermediate temperatures, but rarely in the charge; CA was never found. C3A was a late product. Rarely, ferrite was found only as a late product, being replaced earlier by iron-containing spinel, presumably indicating the existence of reducing conditions at the cooler end of the kiln. Moore concluded that sulphates and chlorides were important mineralizers and that both spurrite and c,A,S played parts in the normal reaction sequence. Later results probably do not exclude this latter conclusion, but both phases can undoubtedly be undesirable artifacts. Wolter (W14) determined the phase compositions of kiln inlet meals from about 20 plants using cyclone preheaters, usually with precalciners. XRD showed that the decomposition products of the clay minerals were especially reactive, though some reaction of quartz also occurred. Phases detected, and some notable absences, were as follows: (1) Unreacted phases from the raw meal: calcite, quartz, minor mica and, felspars, traces of clay minerals. (2) Product phases: free lime, periclase, poorly crystalline belite (p-C,S), ferrite, C12A7 assuming that enough A1,03 was present, and often spurrite. Gehlenite was possibly present, but if so, was minor compared with belite. Less basic silicates and aluminates (e.g. CA, CA,) were absent. (3) Condensed volatiles: arcanite (KS), sylvine (KCI), aphthitalite (rare), NaCl (exceptionally), anhydrite (for high S03/alkali ratios, and then
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less than expected). Absences included thenardite, calcium langbeinite, alkali carbonates, C,S,S and C,A,S. (4) Clinker dust and coal ash: alite, pfa (rare). (5) Secondary products formed on storage or treatment: CH, hemihydrate. The quantitative composition varied widely, depending on the degree of calcination, heterogeneity of the raw meal, and whether kiln dust or volatiles were recirculated. C,,A, and ferrite formed easily. At high degrees of calcination, virtually all the AI,O, and Fe,O, had reacted, 10-60% of the total SiO, content was present as belite or spurrite and the content of free lime was 5 4 0 % . The proportion of the CaO released from the calcite that was combined in other phases was high at low degrees of calcination, and decreased as the latter proceeded; at 40-70% calcination, it was 30-75%, or in rare cases over 90%.
3.3.4 Reaction mechanisms The mechanisms of the reactions in cement raw meals (C9,16,Cl I ) and limesilica composites (W15) have been studied using light and electron microscopy, XRD, high-temperature XRD and high-temperature light microscopy, EPMA and thermal analysis, and free lime determinations. Below about 1200•‹Creactions occur largely in the solid state, the first visible effect in cement raw meals being the appearance of reaction rims around the larger quartz grains, which later become replaced by clusters of belite crystals. Chromy (C9) found that the rims comprised zones of cristobalite, isotropic material of refractive index 1.515-1.525 and (furthest out) belite. By 1200•‹C, the belite layer was up to 2-3 pm thick, and the isotropic material, which formed a thicker layer, was tending to break through, to form necks linking adjacent quartz grains. The isotropic material contained calcium and silicon, and had Ca/Si < 1.0; it gave a diffuse XRD peak at 0.37nm and was probably a glass. XRD and optical evidence also showed the presence of wollastonite and a little tridymite; in samples cooled from 1280•‹C,both were detectable optically, as needles dispersed in the amorphous material and as crystals on the border of the latter with the quartz, respectively. The wollastonite had presumably formed from a-CS on cooling. At 1200-1300•‹C,high-temperature light microscopy showed movement of the silica cores and CS crystals; the amorphous material was softening. Where the belite shell had burst, clusters of belite crystals with a central pore usually replaced the original quartz grain, but where it had not, these clusters only formed at a higher temperature and were more compact, with very little interstitial material. Results at higher temperatures are described in Section 3.4.4.
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Rapid increase in temperature is desirable at temperatures below those at which substantial liquid formation occurs (C9.B27,S21,C1 I.W9,G26). Most of the belite, and almost all of the other product phases, subseq~~ently either melt or react in the presence of the melt, and there is no merit in promoting crystal growth or removal of imperfections, which would impede these processes. Slow heating may also allow the decomposition products of the clay minerals to transform into less reactive phases. I t can also lead to the formation of microstructures unfavourable to the later reactions; Chromji (C9) found that it allowed the belite shells around the silica particles to thicken, producing composites slow to react with lime. In contrast, rapid heating increases movement of the liquid phase, when this forms, and thus improves the mixing of the calcareous and siliceous constituents (CI I).
3.3.5 Condensation or reaction of volatiles Bucchi (B28) reviewed the cycling of volatiles in kilns and preheaters. Condensation or reaction of species from the gas stream takes place below about 1300•‹C.The sulphate and chloride phases produced were noted in Section 3.3.3. Some of the material thus deposited is liquid, even at quite low temperatures: melting in the NS-KS-CS-KCI system begins below 700•‹C (R9). These low-temperature melts have both good and bad effects. Deposition on the feed can promote reactions, as noted earlier. Deposition in the preheater or kiln can cause serious obstructions, and condensation of volatiles is responsible for the formation of kiln rings and deposits in preheaters at temperatures below 1300•‹C(A2,S22,S23,B28,C12). Dust particles become coated with films of liquid condensate which cause them to adhere to obstacles or cooler surfaces. The temperature within a coating or deposit gradually falls as more material is deposited; this allows the formation of compounds within the body of the coating that differ from those present on the hot face. Spurrite, if formed, reinforces deposits by producing a mass of interlocking crystals; C,A,S and c,s,S behave similarly. The kiln atmosphere is high in CO,, and spurrite is readily formed on heating either clinker (A2) or raw meal (S22) in CO, at 750-900•‹C. It decomposes by 950•‹C(A2). In SO,, either raw meal or clinker gives anhydrite at 550-1 150•‹C,and silicosulphate at 1050-1 150•‹C;in mixed gases, the reactions with SO, are favoured over those with CO,, and if K,O is present in the solid, calcium langbeinite is formed at 600-900•‹C (A2). Formation of spurrite appears to be favoured by the presence of K,O or chlorides and inhibited by phosphates (S22), which are, however, undesirable constituents of clinker. Fluoride also appears to favour formation of spurrite and silicosulphate.
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Fluorides and compounds of zinc, cadmium, thallium and lead also undergo cycling and can be deposited in preheaters (Kg). Emission of the toxic heavy metals must be avoided. Fluoride phases that have been reported in deposits include KCa,2(S04)2(Si0,),02F(F6) and an apatite phase of K,Ca,(SO,),F (T9.P 14).
3.4 Reactions at 1300-1450•‹C 3.4.1 Quantity of liquid formed The processes described in this section occur in the presence of substantial proportions of liquid. They comprise: ( 1 ) Melting of the ferrite and aluminate phases, and some of the belite. (2) Nodulization. (3) Reaction of the free lime, unreacted silica and some of the belite, to give alite. (4) Polymorphic change of belite to the a form. ( 5 ) Recrystallization and crystal growth of alite and belite. (6) Evaporation of volatiles.
At the clinkering temperature, the principal phases present at equilibrium are alite, belite and liquid. For the pure CaO-C,S-C,A-C4AF system, this phase assemblage will, except at high AI,O,/Fe,O, ratios, be reached by 1400•‹C (Section 2.3.3). Following Lea and Parker (L5), the quantity of liquid formed in a mix of given composition at this temperature may be calculated as follows. (1) All the Fe20, is present in the liquid; hence the percentage of liquid is 100 x Fe20,/(Fe20,),, where Fe20, and (Fe,O,), are the percentages of that component in the mix and in the liquid, respectively. (2) The composition of the liquid is governed only by the AI20,/Fe,O, ratio of the mix, being represented by the point on the C,S-C,S surface where the 1400•‹Cisotherm is intersected by a plane corresponding to the value of that ratio. The intersection of the 1400 C isotherm with the C3S-C2S surface runs near and approximately parallel to the boundaries between that surface and the C,A and C,AF surfaces. Table 3.4 gives liquid compositions for four values of the A120,/Fe203 ratio. (3) The A1203/Fe20, ratios and Fe20, contents in these compositions are related by the empirical equation
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which may be rearranged to give
whence the percentage of liquid in the mix is 2.95A120,
+ 2.20Fe203.
(4) Lea and Parker (L5) corrected for the contributions to the liquid of MgO and alkalis. Assuming that the solubility of MgO in the liquid is 5-6%, and that the percentage of liquid is about 30%, the contribution from MgO was assumed to be the MgO content of the mix or 2%, whichever was smaller. The liquid was assumed to contain all the K 2 0 and N a 2 0 . Table 3.4 Compositions of liquids in equilibrium with C3S and C,S in the CaOA1203-Fe203-SiO, system at 1400•‹C
Weight ratio
Weight percentages
A1203/Fe203
CaO
6.06 2.62 0.94 0.64
56.6 56.4 55.1 53.9
30.3 26.2 18.8 15.3
Fe20.3
sio,
5.0 10.0 20.0 24.0
8.0 7.4 6.1 6.8
Lea and Parker (LS). with corrections supplied by F. M. Lea and noted by Bogue (B79)
Similar calculations were made at other temperatures. At lower temperatures, the relationship differs according to whether the AI,O,/Fe,O, ratio is above o r below 1.38, the value for the C3S-C2S-C3A-C,AF invariant point. Formulae for the percentage of liquid for various combinations of temperature and A1203/Fe203ratio are given below: 3.00AI2O3 + 2.25Fe203 + MgO* + K,O + N a 2 0 1450•‹C 2.95AI2O3 + 2.20Fe203 + MgO* + K,O + Na,O 1400•‹C 6.10Fe20, + MgO* + K,O Na,O 1338•‹C(A1203/F e 2 0 3 3 1.38) 8.20A1203 - 5.22Fe203 1338•‹C(A1203/Fe203< 1.38) MgO* K 2 0 Na,O
+
+
+
+
where MgO* denotes a n upper limit of 2.0% to the term for MgO. Typical percentages of liquid (for a mix with 5.5% A1203, 3.5% F e 2 0 3 , 1.5% MgO and 1.0% K 2 0 ) are thus 24% at 1340•‹C, 26% at 1400•‹C and 27% at 1450•‹C. Such results, omitting the contributions of MgO and alkalis, are conveniently represented as contours on plots of AI20, against Fe,03 (Fig. 3.3). At 1400•‹C,the effects of the two oxides on the quantity of liquid are additive and fairly similar weight for weight. At 133S•‹C,the effects are not additive,
,
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the maximum amount of liquid for a given total content of AI,O, and Fe,03 being obtained at an A1203/Fe203ratio of 1.38.
Fig. 3.3 Weight percentages of liquid at 1338•‹C(full lines) and 1400•‹C(broken lines) in the Ca0-A1203-Fe,03-Si0, system. Banda and Glasser (B30).
The effects of the minor components on the quantity of liquid have, however, not been thoroughly studied. Swayze's results (S10) indicate that the MgO depresses liquidus temperatures by about 50•‹Cand increases the A1203/Fe203ratio in the liquid at the C3S-C2S-C3A-C,AF eutectic to 1.63. Alkalis and sulphate may depress the liquidus temperatures further, but the effects of liquid immiscibility must be considered.
3.4.2 Burnabilities of raw mixes
The 'burnability' of a mix denotes the ease with which free lime can be reduced to an acceptable value in the kiln. For laboratory use, it has usually been defined by either the quantity of free lime present after a specified regime of heat treatment, or by the time of such treatment needed to reduce the free lime content to some specified value, though some investigators have used dynamic heating or other regimes in attempts to simulate kiln con-
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ditions more closely. As Bucchi (B27) and Sprung (S2I) have noted, none of the measures so far proposed has more than comparative significance, as they do not take into account such factors as the atmosphere. movement of material or rate of heat transfer in the kiln. The rate at which the free lime reacts depends not only on characteristics of the raw mix but also on the effects on the latter of the heat treatment received before entering the clinkering zone and the conditions existing in that zone itself. The measures can strictly be justified only for a given kiln and set of operating conditions. Many studies have shown that burnability decreases with increasing LSF or increasing SR. Increase in LSF implies more CaO that has to react, and increase in SR implies less liquid at a given temperature. These relationships have been discussed quantitatively (CI 3). The AR is relevant at low clinkering temperatures, because it then greatly affects the quantity of liquid (Section 3.4.1) and the temperature at which substantial formation of l~quidbegins, but at 1400•‹Cand above its effect is smaller; however, in the extreme case of white cements, the AR is very high and burnability is low. The temperature at which melting begins is also affected by the contents of minor components, such as MgO, and by the addition of fluxes. Burnability is affected not only by the quantity of liquid, but also by its physical properties, some of which are considered in Section 3.6. Burnability is affected by the particle size of the raw mix, and especially by the contents of coarse particles (H20,L9,C14). During calcination, large particles of calcite and of siliceous materials are converted into aggregates of lime crystals and clusters of belite crystals, respectively. The size distributions of these large particles determine the time required during the last stages of clinkering, when the free lime content is falling to its final value. The fine end of any distribution likely to occur in practice is less important. For assessing burnability at 1400•‹C, the proportions of calcite particles larger than 125 pm, and of quartz particles larger than 44 pm. are particularly important (C14). Burnability also depends on the nature, microstructures and intimacy of mixing of the raw materials and on the contents of minor components. Silica present as quartz is less reactive than that present in clay minerals, and a limestone high in silica or silicate minerals is likely to be more reactive than one that is nearly pure calcite. Inadequate mixing of the particles of the, different constituent rocks present in the raw mix has effects broadly similar to those of too high a proportion of coarse particles. Petersen and Johansen (PI 5 ) discussed the mixing of particles from a statistical viewpoint. Several workers have obtained expressions relating burnability to compositional and other parameters (L9,C13-C15,F7). Fundal (F7), defining burnability (C,,,,) by the percentage of free lime remaining after 30 min at 1400•‹C,found that
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where SR denotes silica ratio, S,, and C,,, are the percentages of siliceous particles above 44 pm and of calcareous particles above 125 pm, respectively. and A , is the percentage of acid-insoluble material, assumed to be quartz. This relation was considered to hold for LSF values of 88-100% and silica ratios of 2-6.
3.4.3 Nodulization Nodule formation occurs through the sticking together of solid particles by liquid. Petersen (P16,P17) showed that the nodules grow partly by coalescence and partly by accretion, and developed a theory to predict the size distribution. Timashev (TI) regarded the process as one in which an initially relatively open assemblage of solid particles was compacted through the action of the liquid, initially by rearrangement of particles, and increasingly later by dissolution and crystallization processes. Nodulization requires an adequate proportion of liquid, and is favoured by low viscosity and high surface tension of the liquid and by small particle size of the solid (Tl.PI7). If there is not enough liquid, the outer parts of the nodules become enriched in silicates, some of which form dust (P15). Overburning can have a similar effect (LIO). If the friable skin of large alite crystals remains on leaving the kiln, the clinker has a sparkling appearance. Much is abraded in the kiln, to form a dust that can cause rings or other deposits to form. I t can also reaggregate to form porous, alite-rich lumps, which can incorporate small clinker nodules. Nodulization, chemical reactions and evaporation of volatiles are interdependent. Alite formation and loss of volatiles affect nodulization, and the compaction that the latter entails must affect the kinetics of the former processes. Alite formation thus occurs in a changing environment. Initially, the clinker liquid is not a continuous and uniform medium enveloping the grains of lime, belite and other solid phases; rather, it fills separate pores and capillaries, forming thin films on the particles, and there are large local variations in composition (TI). As compaction proceeds, it becomes more continuous and more uniform.
3.4.4 Formation and recrystallization of alite In the study described in Section 3.3.4, Chromj, (C9) showed that a melt of
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low Ca/Si ratio began to form at 1200-1300•‹C around the original quartz grains, which were being converted into clusters of belite crystals. From I30O0C, a liquid also began to form around the grains of lime. It differed in reflectance from that surrounding the silica or belite crystals and was considered to be of higher Ca/Si ratio. Where it bordered on the belite clusters, alite was precipitated, initially as a compact layer. This later recrystallized, allowing the liquids to mix. Alite now formed more rapidly; the belite clusters contracted, and the crystals that remained recrystallized. In further studies, C h r o m i and co-workers (CIl,C15,C16) distinguished three stages in the reaction at clinkering temperatures. In the first, melt and belite are formed. In the second, belite continues to form; this stage yields a material consisting of isolated clusters of lime crystals having thin layers of alite on their surfaces and dispersed in a matrix of belite crystals and liquid. In the third, the clusters of lime react with the surrounding belite to give alite, by a mechanism based on diffusion of C a O through the liquid. The temperature increases rapidly during the first stage, but the subsequent stages are isothermal. Several workers have discussed the kinetics of the clinker-forming reactions (J5,B31,Tl,CI I,Cl5,Cl6,G26). Because the material is not uniform, different factors may dominate at different times and in different regions ( T I ) but the principal one appears to be diffusion of C a 2 + through the liquid present between the alite crystals in the layers coating the lime clusters (C 16). From the standpoint of 28-day strength, the optimum size of the alite crystals appears to be about 15 pm, larger crystals containing fewer defects and thus being less reactive (L10). Ono (07) considered that cannibalistic growth ofalite crystals was unimportant, and that the size depended essentially on the rate of heating to the clinkering temperature; if this was low, the alite formed at relatively low temperatures as large crystals, and if it was high, it formed at higher temperatures as smaller crystals. This view has been only partially accepted by other workers, Hofmanner (H21), Long (L10) and Maki and co-workers (M29,M30,M5) all having found that alite crystals grow larger through recrystallization with increase in either time or temperature in the clinkering zone, and even (H21) during cooling. The rate of heating to the clinkering temperature is thus only one factor affecting alite size. In practice, few production clinkers contain alite with an average size as low as 15 pm. Long (L10) noted that alite size is indirectly affected by both bulk composition and fineness of the raw feed. Increases in LSF, SR or AR all make a clinker harder to burn, which tends to increase the size, though with j increased SR or AR, the effect is counteracted by decreases in the quantity i and mobility of the liquid, both of which tend to decrease the size. The ':
1
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presence of coarse particles, especially of silica, also demands harder burning and tends to increase the size. Coarse silica particles lead to increased alite size both for this reason and because they yield large, dense clusters of belite crystals, which on subsequent reaction with lime give large crystals of alite.
3.4.5
Evaporation of volatiles; polymorphic transitions; reducing conditions
Miller (M31) summarized factors affecting the evaporation of volatile species in the burning zone. Much of the alkali and SO, is often present as sulphate melts. Alkali sulphates volatilize slowly because of their low vapour pressures, because diffusion within the clinker nodules is slow, and because the nodules do not spend all their time on the surface of the clinker bed. Evaporation is favoured by increased residence time or temperature, decreased size or increased porosity of the clinker nodules. Low partial oxygen pressure also favours evaporation through decomposition to give SO,, 0, and alkali oxides. Chloride favours it through formation of the much more volatile alkali halides. A high partial pressure of H,O favours it through formation and volatilization of alkali hydroxides. SO, present as anhydrite is more easily lost, because decomposition to SO, and 0, occurs, and this is also true of sulphur present in the fuel as FeS, or organic compounds. Alkali present in the silicate or aluminate phases also appears to be more easily lost than that present as sulphates.
Temperature
OC
Fig. 3.4 DTA curve for a white cement clinker raw mix, obtained in air a t 20•‹Cmin- '. From Chromji (C9).
Alite may be expected to form in clinkers as the R polymorph, and belite as a',, a', or a, according to the temperature. Chromy (C9) and Hofmanner (H21) discussed DTA curves of raw meals (Fig. 3.4). The main exotherm is due to belite formation, and the main endotherm to melting. A small
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Cement Chemistry
superimposed endotherm near 1380•‹Cwas attributed to the a',-a transition of the belite, at a temperature lowered by solid solution (C9). Curves for different raw meals show considerable variations. One would expect some loss of oxygen to occur at the burning temperature, but under normal conditions, this appears to be superficial and reversed during cooling. Long (LI 1) summarized the effects of a reducing atmosphere in the burning zone. Decomposition of the alite is promoted, and more aluminate phase can be formed at the expense of ferrite, with consequent acceleration of setting and change in colour of the ground cement from the normal greenish grey to brown. Some of the Fe2+ that is formed replaces C a 2 +in the solid phases, making burning more difficult and decreasing reactivity towards water. Severe reduction can lead to formation of iron(0). The sulphate phases may also be reduced; moderate reduction leads only to formation of SO, without O,, but more severe reduction can give CaS and KFeS,. The fate of the alkalis is not clear, but such clinkers become deliquescent if quickly cooled. For normal cements, the effects of reducing conditions are all undesirable, but for white cements, which contain very little iron, reducing conditions are preferred because they yield a whiter product. Locher (L12) concluded that the bad effects of reducing conditions were avoided so long as the clinker left the kiln at a temperature of at least 1250•‹Cand thereafter cooled rapidly in air; however, reoxidation of Fe2+ in alite below 1300•‹Chas been observed to cause the formation of exsolution lamellae consisting of C,F and belite (LI 1).
3.5
3.5.1
R e a c t i o n s d u r i n g cooling, grinding o r s t o r a g e
Solidification of the clinker liquid: indications from pure systems
In principle, the clinker liquid can behave in any of three ways during cooling, with intermediate possibilities. In the first, equilibrium between the liquid and the pre-existing solid phases (alite and belite) is continuously maintained. This implies the possibility of material transfer between these solids and the liquid in either direction for as long as any liquid remains. We shall call this situation 'equilibrium crystallization'. In the second, the liquid does not crystallize but forms a glass. In the third, the liquid crystallizes independently, i.e. without interacting with the solid phases already present. Intermediate modes include, for example, equilibrium crystallization down to a certain temperature and independent crystallization below it. Lea and Parker (L5) found that, in the pure CaO-A1203-Fe203-SiO,
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system, the compositions of the liquids in equilibrium with CIS and C,S at 1400•‹Clie within the C3S-C,S-C,A-C4AF subsystem if the AR is between 0.7 and 1.7; for these compositions, equilibrium crystallization and independent crystallization lead to identical quantitative phase compositions. If the AR exceeds 1.7, the liquid composition is deficient in CaO relative to the above subsystem; this results in formation of a little C,,A7 at the expense of C3A if crystallization is independent, and in reaction with some of the alite to form belite if it is not. Conversely, for an AR below 0.7, the liquid composition has a surplus of CaO relative to the C,S-C,S-C,A-C,AF subsystem, resulting in formation of a little free lime if crystallization is independent and in reaction with some of the belite to form alite if it is not. Equations were derived for calculating the quantitative phase composition for various cooling conditions (L5,D2,B29).
3.5.2
D o Portland cement clinkers contain glass or C,,A,?
The applicability of these conclusions to production clinkers requires examination. The compositions of the aluminate and ferrite phases differ markedly from C,A and C4AF, respectively (Chapter 1 ) and, because of the presence of minor components, especially MgO, TiO, and alkalis, the compositions of the liquid phase differ from those in the pure system. We shall consider in turn: first, whether glass is formed, and if so, under what conditions; second, whether either C,,A7 or free lime crystallizes from the liquid phase under appropriate conditions; and third, whether there is evidence of either equilibrium or independent cooling. The early literature contains many references to the presence in production clinkers of glass, often in substantial proportions. This view was based partly on observations by light microscopy; however, this method cannot distinguish glass from crystalline solids of the cubic system unless crystals with distinct faces have been formed, nor from crystalline materials of any kind if the crystals are below a certain size. It was also found that if clinkers believed to contain glass were annealed, their heats of solution in an acid medium increased, and this method was used to obtain approximate estimates of the glass content (L13). This evidence, too, is inconclusive, because the same effect would arise from the presence of small or structurally imperfect crystals. Neither quantitative XRD, SEM nor X-ray microanalysis supports the view that modern clinkers contain significant quantities of glass, except in rare cases. Regourd and Guinier (Rl) found that a few clinkers gave a strong, diffuse XRD peak at 30-35" 28 (CuK,), indicating the presence of glass or other amorphous material, but concluded that the amount of glassy
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Cement Chemistry
or microcrystalline phase did not exceed a few per cent. An SEM and X-ray microanalytical study showed that white cement clinkers contained glass if quenched in water, but not if cooled in air (B3). A laboratory study on the cooling of simulated clinker liquids showed that glass, amorphous to X-rays, is formed on quenching small samples in water (BIO). Somewhat slower cooling, from 1300•‹Cto 700•‹Cin a few seconds, gave 'proto-C,A', a poorly crystalline, iron-rich form of that phase; still slower cooling (1300-1 100'C in 6min) gave normally crystalline aluminate and ferrite phases. Material probably identical with proto-C,A was earlier obtained as a non-equilibrium quench growth (L13), and appears to form easily. The viscosity of the clinker liquid is near the lower limit for glass formation (BIO). These observations suggest that glass may occur in clinkers that have been quenched in water, but is unlikely to be found with slower methods of cooling. Neither X R D nor electron or light microscopy indicates that Cl,A7 is present in any modern clinkers, and when free lime is found, nlicroscopic examination does not suggest that it has formed by precipitation from the liquid. If clinkers of relatively high AR are cooled slowly, the effect of equilibrium cooling is detectable by light microscopy (LI 1). The alite crystals are eroded, and a layer of belite forms on their surfaces; 'pinhead' crystals of belite separate from the liquid. At ARs > 2.5, slow cooling can cause a reduction of up to 10% in the concrete strength, attributable to the lowered content of alite. These effects are not observed at the moderately fast cooling rates more usual in practice. This evidence suggests that under normal conditions crystallization approximates to independent, and that slower cooling is needed to achieve equilibrium crystallization. Production clinkers thus behave differently from mixes in the pure, quaternary system. which at AR > 1.7 give C12A, on independent crystallization.
3.5.3
Evidence from X-ray microanalysis
X-ray microanalyses by many investigators (Chapter 1 ) also provide no indication that C I 2 A 7occurs in normal clinkers. Chose's ((34) results are of particular interest because he did not attempt to distinguish between the individual phases within the interstitial material, of which he thus obtained bulk analyses. Taken in conjunction with the results of other investigations in which individual phases were analysed, they are consistent with the hypothesis that for ARs between about 1 and 4 the interstitial material consists of aluminate and ferrite having the normal compositions given in Table 1.2, together with 4-10% of belite, the amount of which increases with
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AR. For lower ARs, the interstitial material consists essentially of ferrite of higher iron contents, and in white cements it consists of aluminate very low in iron, possibly together with glass. In each case, small amounts of silicate phases are probably also present. The reason why mixes with AR > 1.7 do not yield any C,,A, on independent crystallization is that the solid phases are not pure C3A, C,AF and C2S. For AR = 2.71, the quaternary liquid in equilibrium with C3S, C,S and C3A at 1400•‹Ccontains 55.7% CaO, 27.1 % A1,0,, 10.0% Fe203 and 7.2% SiO, (S8). This composition can be closely matched by a mixture of aluminate (63%), ferrite (30%) and belite (7%) with the normal compositions given in Table 1.2, the bulk composition of this mixture being 54.4% CaO, 26.4% A1203, 9.7% Fe,O,, 5.6% SiO, and 1.8% MgO, with < I % each of TiO,, Mn203,N a 2 0 and K,O. Independent crystallization can thus yield a mixture of the three phases. The liquid composition cannot be matched by a mixture of pure C3A, C,AF and C,S, which is relatively too high in CaO, so that if no ionic substitutions occurred, some C,,A, would also be formed. A strict comparison would be with the actual composition of the clinker liquid, which is modified by minor components, but lack of adequate data precludes this.
3.5.4
Effects of cooling rate on the aluminate and ferrite phases
It follows from the above discussion that the compositions of the aluminate and ferrite phases in typical clinkers are controlled by that of the clinker liquid and the conditions under which the latter crystallizes. For clinkers with AR 1 4 , their compositions are relatively constant because both phases are present, and because cooling rates are normally moderately fast. The compositions must nevertheless depend to some extent on that of the raw mix, especially in regard to minor components. The observation that resorption of alite occurs on slow cooling implies that their compositions. or their relative amounts, or both, depend on the cooling rate. They could also be affected by the clinkering temperature, and the time during which it is maintained. XRD evidence shows that the composition of the ferrite. at least, is affected by the cooling rate (02,B32), but the lack of data on the relation between composition and cell parameters except for compositions in the pure C,(A,F) series precludes detailed interpretation. IR evidence indicates the occurrence of structural and compositional variations in both phases dependent on cooling rate (V3). X-ray microanalyses of the aluminate and ferrite phases in similar clinkers cooled in different ways would provide data of interest. The conditions of crystallization affect the ferrite and aluminate in ways
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Cement Chem~stry
other than in their mean compositions. Slow cooling produces relatively large crystals of each phase, while fast cooling produces close intergrowths; textures are discussed further in Section 4.2.1. Zoning occurs readill, in the ferrite (Section 2.3.1), and may also occur in the aluminate. The distribution of atoms between octahedral and tetrahedral sites in the ferrite depends on the temperature at which internal equilibrium within the crystal has been achieved (Section 1.5.1). The degree of crystallinity of both phases appears to vary with cooling rate (V3). All these effects, and perhaps others. may affect the behaviour of the interstitial material on hydration.
3.5.5
Other effects of cooling rate
The behaviour of MgO in clinker formation depends markedly on the cooling rate (Lll). If a clinker is burned at a high temperature ( > 1500 C), relatively high contents of MgO can enter the liquid, and on rapid cooling, much of it remains in the aluminate and ferrite phases and only a small quantity of periclase separates as small crystals. On slow cooling from high temperatures, only around 1.5% of MgO (referred to the clinker) is taken into solid solution, the excess forming large periclase crystals. At temperatures below 1450•‹C,the MgO is less readily taken into solution. and clusters of periclase crystals can remain; the crystals grow with time and even with temperature in this range. The polymorphic transitions of the alite and belite that occur during cooling were discussed in Chapter 1. The belite of normal clinkers contains sufficient proportions of substituent ions to prevent dusting to give the yform; the mechanical constraint provided by the surrounding material may also contribute. The cooling rate affects the way in which the belite transitions occur, with effects recognizable microscopically (02). Cooling rate is also reported to affect alite crystal size, rapid cooling giving smaller crystals, which are more easily ground (S24). If a clinker is cooled too slowly in the region from 1250LCto about 1 10O0C, the alite begins to decompose. An intimate mixture of belite and lime forms as pseudomorphs after the alite, and the decomposition is especially likely to occur if the clinker has been fired under reducing conditions (LI 1). Mohan and Glasser (M32) reviewed and extended knowledge of the kinetics of this process. The kinetics follow a sigmoidal curve, in which an initial induction period merges into an acceleratory phase. followed by a deceleratory one as the process nears completion. These results can be explained by assuming that the rate is controlled by nucleation of the products on the surfaces of the C,S grains; in accordance with this. the reaction is accelerated, and the induction period largely eliminated, if the
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C,S is sufficiently intimately mixed with either C2S or CaO. Some substituent ions affect the rate: A13+ has little effect, Fe3' markedly accelerates. ~ a slightly ' retards, and Mg2+markedly retards decomposition. Where both ~ g * and ' Fe3+ are present, the accelerating effect of the Fe3' is dominant. The decomposition is also accelerated by water vapour and, very strongly. by contact with sulphate melts; with a CaS0,-Na2S0, melt, decomposition was complete in 2 h at 1025•‹C. All the effects described above indicate that rapid cooling is desirable: the aluminate phase reacts more slowly with water when finely grained and intimately mixed with ferrite, making it easier to control the setting rate (S24), decrease in alite content either from reactions involving the interstitial material or from decomposition is avoided, a higher MgO content can be tolerated, and the clinker is easier to grind.
3.5.6 Crystallization of the sulphate phases Much of the SO, is present at the clinkering temperature in a separate liquid phase, immiscible with the main clinker liquid. The alkali cations are distributed between the two liquids and the alite and belite. During cooling. some redistribution of alkali cations and sulphate ions between the liquids may be expected to occur, the sulphate liquid finally solidifying below 900 C to yield alkali or potassium calcium sulphates. In most Portland cement clinkers the K,O and N a 2 0 are in excess of the equivalent amount of SO,, and K 2 0 is usually more abundant than Na20. Early work, reviewed by Newkirk (N9). indicated that the SO, combined as alkali sulphates, the excess of alkalis entering the silicate and aluminate phases. Sulphate phases shown or believed to occur in clinkers were potassium sulphate and aphthitalite. Subsequently, calcium langbeinite (c,KS,) and anhydrite were also found, and Pollitt and Brown (P2) made a systematic study of the distribution of alkalis and SO, in a number of worksand laboratory-prepared clinkers. Fig. 3.5 and the following discussion are based on their conclusions and the tabulated data in their paper. In this work, the water-soluble alkalis and SO, were those present in alkali sulphates or C,KS,, the remaining fractions of these components being present in anhydrite or in the silicate or aluminate phases. The distributions of all three components are largely controlled by the molar ratio of total SO, to (total K 2 0 -t total Na20) in the clinker, especially for the important range of this ratio below 1 .O. K 2 0 has a greater tendency than N a 2 0 to combine with SO,, the fraction of the total K 2 0 that is water soluble being consistently about double the corresponding quantity for Na,O.
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Cement Chemistrv
1 0 0.8 0.6 aJ
0.4
d
13 7 d
0.2 L
aJ
c, 0 . 0 3 C 0 .
-
+ U
m L
LL
1.0 0.8 0 6
0
0 2
0.6 0.8 1.0 1.2 1.4 1 6 Molar r a t l o S 0 3 1 ( K 2 0 + N a 2 0 )
0.1,
+
1 8
7 0
Fig. 3.5 Fractions of the K 2 0 , Na20, K 2 0 N a 2 0 (molar) and SO, that are water soluble, plotted against the molar ratio of total SO, to (total K?O total N a 2 0 ) for production clinkers (circles) and laboratory clinkers (squares). Recalculated from the data of Pollitt and Brown (P2).
+
+
For S 0 , / ( K 2 0 N a 2 0 ) molar ratios below about 0.5, the contents of total SO,, water-soluble SO, and water-soluble K 2 0 + N a 2 0 are approximately equal, showing that virtually all the SO, occurs as alkali sulphates, which also contain up to 55% of the K 2 0 and 27% of the N a 2 0 . For ratios between about 0.5 and 1.0, almost all the SO, is usually still water soluble, but the content of water-soluble K 2 0 + N a 2 0 falls below that of watersoluble SO,, showing that C,KS, is also formed. For ratios above 1.0, appreciable fractions of the SO, are not water soluble, and thus occur in the silicate or aluminate phases or as anhydrite. The fractions of the K 2 0 and N a 2 0 that are water soluble approach 1.0 and 0.5, respectively. at ratios around 1.5; at higher ratios, they are somewhat erratic and show some tendency to decrease again. This decrease is unexplained, but in other
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respects these results are broadly consistent with those of the phase equilibrium studies on pure systems described in the previous section. From the smoothed curves in Fig. 3.5, and given the total contents of K,O, Na,O and SO, in the clinker, one may readily estimate the probable water-soluble contents of these components, and thence, making certain assumptions, the contents and compositions of the individual sulphate phases. The curves may also be represented by empirical equations (Section 4.4.3). Based on the data in the preceding section, we shall assume that water-soluble SO, not balanced by water-soluble K 2 0 and N a 2 0 occurs as C,KS,. and the remaining water-soluble K 2 0 and Na,O as aphthitalite if the K/Na ratio is within the appropriate range, or as aphthitalite of the appropriate end member composition mixed with KS or NS if it is not. As an example, for a clinker with 0.80% K 2 0 , 0.30% Na,O and 0.60% SO,, this gives: water-soluble K 2 0 , 0.50%; water-soluble N a 2 0 , 0.09'/ o ; watersoluble SO,, 0.60%; C,KS,, 0.1%; K,-,N~,S, 1.0% and s = 0.25; KS, 0.1%. For most normal clinker compositions, the major sulphate phase will be aphthitalite with its maximum K/Na ratio of 3.0, with minor amounts of KS or C,KS, or both; only with unusually low values of K/Na in the clinker would one expect NS to be formed. It is only possible to estimate a maximum value for the content of anhydrite, based on the amount of SO, that is not water soluble; this will often be zero, as in the example given above. Anhydrite may, however, be determined by QXDA (Section 4.3.2). Rarely, the amount of water-soluble SO, is insufficient to balance that of water-soluble alkalis; this has been attributed to the presence of traces of alkali carbonates (P2), or of potassium oxide and potassium aluminate (KA) F 4 ) . 3.5.7
Changes during grinding or storage
Meric (M33) reviewed the grinding and storage of clinker. Factors influencing the ease with which a clinker can be ground include the size distribution of the nodules, the texture and phase composition, the cooling rate and the conditions under which it has been stored. Grindability increases with alite content and decreases with that of the interstitial phases. Hornain and Regourd (H22) defined a brittleness index, which they showed to have values of 4.7 for alite, 2.9 for aluminate, about 2 for ferrite and 1.8 for belite. They also found that microcracking produced by rapid cooling from 1250•‹C increased grindability. Some specifications allow the use of grinding aids (Section 11.3.2). The particle size distributions of cements are discussed in Section 4.1.
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The heat produced on grinding can cause partial conversion of the gypsum into hcniihydrate (2CaS04.H,0) or y-CaSO,. The extent to which this occurs depends on the temperature reached and the relative humidity within the mill. The conversions affect the rate at which the calcium sulphate dissolves on hydration. Partial conversion Lo heniihydrate may be desirable, as the water present in gypsum can cause the particles of cement to adhere during storage, with formation of lumps (L12). T o obtain an optimum rate of dissolution on hydration, part of the gypsum may be replaced by anhydrite. Lumpiness on storage can also result from the reaction of gypsum and potassium sulphate to form syngenite (CKS,H) (LI I ) . Maultzsch c.t ril. (M34) reported on the effects of prolonged storage; minor amounts of gypsum (from hemihydrate), syngenite. calcite and normal hydration products were present.
3.6
Effects of minor components
3.6.1 General Minor or trace components derived from raw materials, fuel, refractories or other plant materials, or added deliberately, can affect the reactions of clinker formation, or the properties of the product, o r both. Their effects can be beneficial or harmful. Beneficial effects include acceleration of the clinkering reactions or lowering of the temperature at which they occur. or increase in the reactivity of the product leading to more rapid strength development. Harmful effects include decrease in alite content, volatilization in the kiln with consequent formation of kiln rings or other deposits, decrease in the durability of concrete made with the cement, or the introduction of poisonous elements. Some elements have beneficial effects at IOU. concentrations and harmful ones at higher concentrations. Bucchi (B33.B28) reviewed some of the effects on the manufacturing process. The main clinker liquid can be affected through alteration in the temperature at which it begins to form o r in its quantity or properties at a given temperature. A flux is an agent that promotes a reaction by increasing the quantity of liquid at a given temperature; Al,O, and Fe,O, are fluxes for the formation of alite. Butt, Timashev and co-workers studied the properties of clinker liquids (B31,TI). They found that the viscosity and surface tension of the liquid are markedly affected by relatively small additions of other ions. Species containing strongly electropositive elements increase the viscosity and ones containing strongly electronegative elements decrease it. This was attributed to changes in the distribution of A13+ and Fe3+ between octahedral and tetrahedral coordination. The effects of adding more than one minor
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component are not additive, and can be conlplex, e.g. alkali sulphates decrease the viscosity, but even a small stoichionietric excess of alkali oxides over SO, increases it. Many species lower the surhce tension, because ions that do not easily fit into the structure of the liquid tend to become concentrated near to an interface, but the situation may be complicated by liquid immiscibility. Viscosity has a particularly important effect, a low value favouring alite formation by accelerating dissolution of lime and belite and diffusion through the liquid. Decrease in surface tension may be expected to favour wetting of the solids and penetration of the liquid into porous aggregates of lime crystals. Effects of minor components on the solid phases are of various kinds. A mineralizer is an agent that promotes the formation of a particular solid phase by affecting the equilibria through incorporation in one or more of the solid phases. CaF, acts both as a mineralizer and as a flux in promoting the formation of alite. The mechanism of an observed effect is often not clear, and perhaps for this reason, the term 'mineralizer' is often used loosely. Other examples of action of minor components on solid phases include the stabilization of M, C,S and P-C2S at room temperature, the modification of the aluminate structure resulting from incorporation of alkali, and the nonformation of C,,A, in situations where it would be formed in the pure CaOAl,O,-Fe20,-SiO, system. Minor components can affect the properties of a clinker phase in other ways, such as by causing changes in crystal size, morphology or structural perfection, and can also modify the microstructure of the clinker as a whole. The first I I minor components of raw mixes, in sequence of decreasing median concentration, are MgO, K 2 0 , SO,, Na,O, TiO,, Mn,O,, P,O,, SrO, fluorides, chlorides, and Cr20, (B27).
3.6.2 Effects o f s-block elements The effects of K 2 0 and Na,O in producing low-temperature melts and volatile species have already been discussed. K,O and Na,O lower the temperature of formation of the melt but, in the absence of SO,, increase its viscosity. If present in sufficient concentration, they lead to formation of orthorhombic or pseudotetragonal aluminate; this can have undesirable effects on the rheology of fresh concrete. High K,O contents can lead to syngenite formation and consequent lumpiness on storage. High contents of either K,O or Na,O can lead to destructive reactions with certain aggregates (Section 12.4).Alkalis increase early strength but often decrease late strength (Section 7.6.3). MgO also lowers the temperature at which the main melt begins to form
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and increases the quantity of liquid; i t slightly decreases the latter's viscosity (B31,Tl). Its presence is probably the basic cause of the lion-forniation of C,,A, in normally cooled clinkers of high AR. I t stabilizes M , alite (Section 1.2.4). I t retards the decomposition of alite during cooling. These elrects are beneficial, but the formation of periclase, discussed in Section 3.5.5, limits the permissible content, which most national specifications set at 4% or 5%. Some limestones contain substantial proportions of SrO. Small contents favour alite formation, but phase equilibrium studies show that it increases the range of compositions at which free lime, with SrO in solid solution, is present at the clinkering temperature (B34,G27,G28). This limits the permissible content of SrO in mixes of normal LSFs to about 3%.
3.6.3 Effects of p- and d-block elements The behaviour of CaF, as a flux and mineralizer was described in Section 2.6.1. In addition to promoting formation of alite at markedly lower temperatures, CaF, accelerates the decomposition of calcite. Volatilization does not cause serious problems in the kiln, and it has often been added to raw meals, typically in proportions below 0.5%. I t retards setting, possibly through formation of C,,S,.CaF, during cooling, and this limits its use. As with admixtures to the raw meal in general, i t is also necessary to consider whether the resulting savings outweigh the extra costs. This is most likely to be the case with raw materials that would otherwise require particularly high burning temperature. Chloride contents must be kept low to avoid corrosion of steel in reinforced concrete (Section 12.3) and formation of kiln rings and preheater deposits. Contents below 0.02% are preferred. though higher ones can be acceptable if a sufficient proportion of the kiln gases is bypassed or in less energy-efficient (e.g. wet process) plants. The effects of SO, include formation of alkali sulphates and other low melting point phases and volatile species, discussed earlier in this chapter and in Section 2.5.2. Although much of the SO, is present in a separate liquid phase, the main melt begins to form at a lower temperature and its viscosity is reduced, rendering CaSO, an effective flux. Against this, it leads to the stabilization of belite and lime at clinkering temperatures, and consequent decrease in content or even disappearance of alite, though this effect is lessened if MgO is present (G16). Sulphate can be removed by increasing the burning temperature, but the inclusions of lime in alite that have formed at lower temperatures are not easily lost (L14). Portland cement clinkers typically contain around 0.2% of P,O,; higher contents lead to decreased formation of alite, and if above about 2.5% to
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presence of free lime. An early conclusion that a-C,S forms a continuous series of solid solutions with a ('super a') C3P above 1450•‹C(N12) was modified by later work, which showed that a miscibility gap exists at 1500•‹C (G29). Solid solutions also exist between other combinations of C,S and C3P polyniorphs, lowering the temperatures of the C,S transitions and. for some compositions, stabilizing a or a' forms at room temperature. In the CaO-C2S-C,P system (G29), C3S has a primary phase field, but at a clinkering temperature of 150OUC,mixes with more than a few per cent of p20, do not yield C3S. The tolerance for P,O, is somewhat increased if Fis also present (G21). The 3d elements from titanium to zinc all lower the viscosity of the clinker liquid in varying degrees, but their effects on clinker formation and properties present a confusing picture. I t has been claimed that the content of alite in clinkers decreases with increase in that of TiO,, but the effect is much less marked if the latter is regarded as replacing SiO, (B27). Chromium can enter the clinker from raw materials or refractories; cements high in it have been reported to give high early strengths, but toxicity renders their use impracticable. The enhanced early strength is possibly due to high defect concentrations in the alite (B27), in which the chromium occurs as Cr4+ (56).Both manganese and titanium occur in their highest concentrations in the ferrite phase, and probably for this reason yield darker coloured clinkers. The cycling of zinc, cadmium, thallium, lead, sulphur, fluorine and chloride was noted in Sections 3.2.5. 3.3.5 and 3.4.5.
Properties of Portland clinker and cement 4.1
4.1 .I
Macroscopic and surface properties
Unground clinker
This chapter deals with chemical and physical properties other than ones for which the nature of the hydration products must be considered, which are treated in Chapters 5 to 8. In general, properties of the whole clinker or cement are alone considered, those of the constituent phases having been dealt with in Chapter I, but factors affecting the reactivities of these phases are included as a link with the following chapters on hydration. Portland cement clinker emerges from a dry process kiln as rounded pellets, or from a wet process kiln as irregularly shaped lumps, in either case typically of 3-20 mm dimensions. Typical clinkers are greenish black, the colour being due to ferrite phase that contains M g 2 + (M 18); in the absence of M g 2 + , they are buff. Reducing conditions in the kiln typically produce clinkers that are yellowish brown, especially in the centres of the lumps, where less reoxidation has occurred. Light colours can also arise from underburning. The bulk density of a clinker, determined under standard conditions by rejecting material passing a 5-mm sieve, pouring the remainder into a conical container, and weighing, is called the litre weight. This quantity, which is typically 1.25-1.35 kg I - ' . gives some indication of the operating conditions. Its optimum value depends on the composition. The measured true density of clinker is typically 3 150-3200 kg m-3; the value calculated from the X-ray densities and typical proportions of the individual phases is about 3200 kgm-! The difference is probably due mainly to the presence of pores inaccessible to the fluids used in the experimental determination. Using mercury porosimetry. Butt et ul. (B35) determined pore size distributions and discussed their relations with burning and cooling conditions and grindability.
4.1.2
Particle size distribution of ground clinker or cement
Allen (A3) d~scussed methods suitable for determining t h ~ simportant 96 1
Properties of Portland Clinker and Cement
97
property. Sieving is suitable only for determining the proportions of particles in ranges above about 45 pm. In airjet sieving, jets speed the passage of material through the sieve and prevent the mesh from becoming blocked. For the entire range of size, the most widely used methods are ones based on either sedimentation in liquids or diffraction of light. In the X-ray sedigraph, the progress of sedimentation is monitored by the absorption of an X-ray beam and the results calculated using Stoke's law. Because highly concentrated suspensions are used, there is some doubt as to whether this law is strictly applicable. An older method employs the Andreason pipette, by which samples are withdrawn from a suspension at a fixed depth after various times, so that their solid contents may be determined. In laser granulometry, light from a laser passes through a suspension and the particle size distribution (PSD) is calculated from the resulting diffraction pattern. Light microscopy can provide information on the distribution of both particle size and particle shape, and SEM with image analysis (Section 4.3.1) can, in principle, provide much additional information.
Fig. 4.1 Rosin-Ramrnler plot of the particle size distribution of a typical Portland cement. S = specific surface area attributable to particles of size smaller than x. Open circuit grinding; based on the data of Surnner et ul. (S25).
With all the methods, complications o r uncertainties of interpretation arise due to effects associated with particle shape and with flocculation, so that the results given by different methods d o not necessarily agree and usually have relative rather than absolute significance. Fig. 4.1, curve R, shows the cumulative PSD, obtained using an X-ray sedigraph, for a
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Cement Chemistry
typical Portland cement ground in an open circuit mill (S25). In modern ordinary Portland cements, some 7-9% of the material is typically finer than 2 pm and 0 4 % coarser than 90 pm. Meric (M33) discussed analytical expressions used to represent the PSD of cements. Probably the most successful is the Rosin-Rammler function, which may be written in logarithmic form as In(IOO/R) = n (In s - In s o )
(4.1)
where R is the weight percentage of material of particle size greater than s. The constants xo and n describe the distribution; xo is a measure of average size, 36.79% of the material being of size greater than so, and n is a measure of breadth of distribution, increasing as the latter becomes narrower. For the example shown in Fig. 4. I, x, is 22 pm and n is 1.03. For open circuit milling, the values of n obtained using an X-ray sedigraph are 0.99-1.07; for closed circuit milling, in which the fine material is separated and only the coarser material further ground, n can be as high as 1.23-1.28 if the separation is efficient (S25). A narrow PSD can also be obtained using a vertical spindle mill. Laser granulometry typically gives values of tt lower by 0.15-0.20 (S25).
4.1.3 Specific surface area determination
The specific surface area of cement is commonly determined directly by air permeability methods. In the Lea and Nurse method (L15). a bed of cement of porosity 0.475 is contained in a cell through which a stream of air is passed, and steady flow established. The specific surface area is calculated from the density of the cement, the porosity and dimensions of the bed powder, the pressure difference across the bed, and the rate of flow a kinematic viscosity of the air. In the Blaine method (B36), a fixed volume o air passes through the bed at a steadily decreasing rate, which is controlled and measured by the movement of oil in a manometer, the time required being measured. The apparatus is calibrated empirically. most obviously using a cement that has also been examined by the Lea and Nurse me The two methods gave closely similar results. The Blaine method, th not absolute, is simpler to operate and automated variants of it have been devised. Other methods that have been used to determine specific surface areas of cements include the Wagner turbidimeter (W16) and BET (BrunauerEmmett-Teller) gas adsorption. The former, as conventionally used, gives very low results because of a false assumption that the mean diameter of the particles smaller than 7.5 pm is 3.8 pm, which is much too high. The BET method gives results two to three times higher than the air permeability
Properties of Portland Clinker and Cement
99
because it includes internal surfaces present in microcracks or in pores open at only one end. The specific surface area, like the PSD, is thus a quality whose value depends on how i t is defined, and is liable to be affected by any pretreatment or conditions affecting the degree of flocculation. In practice, air permeability methods are widely used. Typical values are 300-350m2 kg - ' for modern ordinary Portland cements and 400-450m2 kg for rapidhardening Portland cements. From a PSD curve, one may calculate the specific surface area, S:
-'
where f is the weight fraction of material consisting of grains assumed to have a diameter or edge length (1 (in pm), p is the density in kg m-3, and the result is in mZkg-'; F is an empirical constant which takes into account the differing assumptions regarding surface shape that are made implicity or explicity in the determination of PSD and the definition of specific surface area. This formula applies to both cubic and spherical grains. For a calculation starting from a PSD obtained using an X-ray sedigraph to give a specific surface area comparable to that obtained by an air permeability method, F is typically about 1.13. Fig. 4.1, curve S, shows the contributions to the calculated specific surface area of particles of differing sizes, assuming that particles smaller than 2 pm have a mean size of I pm. About 40% of the total is provided by particles smaller than 2 pm. In principle, the specific surface area may also be calculated from the PSD using analytical expressions derived from such functions as the RosinRammler expression. In practice, this is not always satisfactory because the specific surface area is so highly dependent on the bottom end of the distribution.
4.1.4
Particle size distribution, phase composition and cement properties
Because the constituent phases of a cement are not equally easy to grind, different particle size fractions differ in composition. Gypsum, and its dehydration products, are concentrated in the finer fractions. Osbaeck and J ~ n (08) s concluded that each I % of gypsum contributed about 10 m2 kg- ' to the specific surface area; in a typical case, some 15% of the total specific surface area is thus due to gypsum. The content of alite decreases, and that of belite increases, with increasing particle size (R 12,G30), the contents of aluminate and ferrite phases being little affected. The depth of reaction of the cement grains after a given time has been
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Cement Chemistry
determined by light microscopy; it may be expected to vary with the composition and microstructure of the clinker. Anderegg and Hubbell (A4,A5) found values of 0.43-0.47 pm at I day, 1.7-2.6 pm at 7 days, 3.55.4pm at 28 days and 6.1-8.9 pm at 150 days. Zur Strassen (27) gave somewhat higher values (e.g. 6-7 pm at 28 days). From the depth of reaction and the PSD curve, and assuming the grains to be spherical, one may calculate the fraction, a, of cement that has reacted after a given time: a
=
C f x [l - (1 - 2h/d)3]
(4.3)
where h is the depth of reaction in pm and C, f and d have the same meanings as in equation 4.2 and the quantity in square brackets cannot be less than zero. For the cement to which the data in Fig. 4.1 refer, and using the means of Anderegg and Hubbell's values, this gives 0.28 at I day, 0.63 at 7 days, 0.80 at 28 days and 0.90 at 150 days. These values are in reasonable agreement with those typically found using X R D (Section 7.7.1). Ritzmann (R12) determined the compressive strengths of pastes of a number of cements of known PSD, made by mixing separated fractions. For a given cement and waterlcement (w/c) ratio, the strength was linearly related to the fraction of cement reacted, calculated in this way. At 1 day, the reaction is primarily of the smaller particles, and the fine end of the PSD is therefore the important one for the strength. As the age increases, the coarser part of the distribution becomes progressively more important; by 28 days the particles smaller than about 10 pm have reacted completely, and the content of particles up to about 45 pm in size is of major importance. For a given specific surface area, narrowing the PSD increases the degree of reaction at 28 days, and thus also the strength (L16,S25,K9). The relation may be quantified using equation 4.3. Narrowing the PSD also saves energy in grinding (S25), but increases the amount of water needed to produce a mix of given workability (S25,S26); this latter effect can be compensated by adjusting the temperature during milling so as to increase the extent to which the gypsum is dehydrated to hemihydrate or soluble anhydrite (S25).
4.1.5 Chemical analysis Ground clinker or cement may be analysed by standard methods. such as Xray fluorescence (XRF). Determination of free lime is discussed in Section 4.3.3. With X R F or other methods in which no fraction of the material is excluded from the analysis, insoluble residue (determined separately) is included in the oxide components and therefore should be omitted from the total. It can often be assumed that two-thirds of it is SiO, and the rest
Properties of Portland Clinker and Cement
101
AI,O,. Classical, wet analysis gives inaccurate results for A1,0, unless the effects of P,O, and TiO, are allowed for. Chemical analysis, microscopy, X R D and other methods of examination should be carried out on the same, representative sample of material. A procedure for obtaining such a sample is described in the next section.
4.2 4.2.1
Light microscopy
General
Light microscopy provides much information on clinker microstructure and thereby on the conditions existing at various stages of the manufacturing process. Much the most widely used technique has been the examination of polished and etched sections in reflected light, and the following sections refer to this unless otherwise stated, but additional information has been obtained from the examination of thin sections or powder mounts in transmitted light. Fuller accounts are given elsewhere (G31,H2l,LlO,C17). It is vital to obtain a representative sample; the following procedure (L10) is typical. If a kiln is known to give a reasonably consistent product, samples taken at intervals over a suitable period are aggregated, but if not, they are examined separately. Each sample examined comprises at least 15 kg of clinker, and is first examined visually t o distinguish kiln deposits or refractories. It is then crushed to below 6 mm and a representative sample obtained, e.g. by quartering. The 2-4 mm fraction of this sample is used. Ten to twelve samples of uncrushed nodules, covering the range of nodule sizes. are also examined, together with a sample of the material smaller than 2 mm, if much of this is present in the original clinker. The relative amounts of nodules of different sizes, and of the material below 2 mm, are noted. For examination in reflected light, the sample is mounted in a plastic resin and is cut and polished using only non-aqueous lubricants. The most generally useful etchant is probably H F vapour, which has the merit of not removing alkali sulphates; the slide is inverted over a vessel containing 40% aqueous HF for 10-20 seconds. Treatment with water followed by 0.25% HNO, in ethanol and finally ethanol is also widely used, and many other etchants have been employed for specific purposes. The etchants produce thin films of decomposition products, which yield interference colours when viewed in reflected light. Fig. 4.2 shows a polished and etched section of a production clinker. Crystals of alite and belite are embedded in a matrix of aluminate and ferrite phases. If H F vapour is properly used, the alite is yellowish brown, the belite is blue o r red, and the aluminate is grey; the ferrite, which is unattacked, is
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Cement Chemistry
brightly reflective. The alite crystals are angular, often pseudohexagonal: the belite crystals are rounded and normally striated, as described in Section 1.3.2. Ideally, the alite crystals are 15-20 pm in average size, though in practice they are usually much larger, and there should be no clustering of either alite or belite. The average size of the belite crystals is typically 2540 pm.
Fig. 4.2 Reflected light micrograph of a polished and etched section of a Portland cement clinker, showing crystals of alite (dark, angular) and belite (less dark, rounded) embedded in a matrix of interstitial material, itself composed mainly of dendritic ferrite (light) and aluminate (dark). Courtesy Materials Science Department, British Cement Association.
A good clinker should have been rapidly cooled. and this will cause the ferrite and aluminate phases to be mixed on a scale of a few micrometres, the ferrite often forming dendrites or needles in a matrix of aluminate. In transmitted light, the ferrite phase is pleochroic. The normal, cubic aluminate tends to fill the spaces between the other phases and thus to exhibit no definite form; it is isotropic in transmitted light. In clinkers with an excess of alkali over SO,, cubic aluminate may be partly or wholly replaced by the orthorhombic variety. This forms relatively large, prismatic crystals, which occur in a matrix of the other interstitial phases and are birefringent in transmitted light.
Prooerties of Portland Clinker and Cement
103
There should be little free lime. What there is should occur as rounded grains, typically 10-20 pm in size, and associated with alite and interstitial material. Lime appears cream in sections etched with H F vapour. Its presence may be confirmed by a microchemical test using White's reagent ( 5 g of phenol in 5 ml of nitrobenzene + 2 drops of water); long, birefringent needles of calcium phenate are formed. The test also responds to CH. Alkali sulphates occur in the clinker pore structure; they are etched black with H F vapour, and inhibit the etching of silicate phases with which they are in contact. The phases may be determined quantitatively by point counting (H21 ,C 18). The areas of phases on a polished section of a cement clinker are proportional to the volumes in the material. Random and systematic errors arise from imperfect sampling, counting statistics and incorrect identifications of phases by the operator. Error from counting statistics depends on the number of points, the size of the grid and the grain sizes of the phases; 4000 points normally gives reasonable precision (H21). Systematic errors from incorrect identification of phases generally exceed errors from counting statistics, and give rise to differences in results between operators (C18). With many clinkers, it is impracticable to determine the aluminate and ferrite phases separately by this method.
4.2.2
Effects of bulk composition, raw feed preparation a n d ash deposition
The bulk composition affects the relative amounts of phases, the LSF, S R and AR affecting primarily the ratios of alite to belite, silicate phases to interstitial material and aluminate to ferrite phase, respectively. An LSF that is too high, either absolutely or in relation to the burning conditions, also gives rise to an excessive content of free lime. Belite may occur in such clinkers only as inclusions in alite. Inadequate grinding of the raw meal is shown by the presence of large clusters of free lime or belite, up to several hundred micrometres in size and with defined outlines reflecting those of the original particles of calcareous or siliceous material, respectively. Coarse fragments of dolomitic limestones similarly yield localized concentrations of periclase, usually intimately mixed with lime. Periclase crystals are characteristically angular, up to about 20 pm in size, and are unattacked by normal etchants. Due to their hardness, they stand out in high relief in unetched sections. Less sharply defined segregation indicates inadequate mixing, and is typically shown by the presence of regions rich in belite and in alite plus lime, in each case together with interstitial material. In extreme cases, entire nodules can depart seriously from the mean composition.
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Cement Chemistry
Incomplete assimilation of coal ash is shown by the existence of large regions high in belite, and containing also interstitial phases. The locations of these regions indicate the stage of the burning process during which the ash was deposited; if i t was late, the belite-rich material forms coatings on clinker nodules, in which small nodules can be encapsulated. whereas if it is deposited earlier, it can form part of the kiln coating, which may later break away and form the centres of nodules. If the ash contains unburnt matter, localized reducing conditions can occur. Insufflation of materials into the kiln can produce similar effects (L10).
4.2.3
Effects of burning conditions and cooling rate
Underburning is shown by the presence of abundant free lime and a low content of alite, the crystals of which are very small ( < 10 pm). The average size and other characteristics of the alite and belite crystals provide much further information about the burning conditions; factors governing them were discussed in Chapters 1 and 3. Ono ( 0 7 ) considered that alite size was an indication of the rate of heating to the clinkering temperature, which if high led to the formation of smaller crystals, but other studies indicate that many other factors are involved, and that overburning, in particular, can cause increase in size through recrystallization (Section 3.4.4). Another indication of excessively hard burning is the presence of material high in large alite crystals and low in interstitial material, formed by the withdrawal of the liquid into the centres of the clinker nodules (L10). The effects of slow cooling (Section 3.5) are readily detectable by light microscopy. The most general are coarsening of the texture of the interstitial material and a change in the belite, when viewed in transmitted light, from colourless to yellow. The belite crystals may also develop ragged or serrated edges. Slow cooling can also cause resorption of alite, with deposition of small crystals of belite as fringes on the alite and in the body of the interstitial material, increase in alite crystal size and, if it occurs below 1200•‹C,decomposition of alite to an intimate mixture of lime and belite. The effects of reducing conditions (Section 3.4.5) are similarly detectable by light microscopy (LI 1 ,L14).
4.2.4 Applications of light microscopic investigations Light microscopy can be used to find the causes of unsatisfactory clinker quality or to determine what modifications in composition or plant operation are needed to change the clinker properties in a desired direction. It
Prooerties of Portland Clinker and Cement
105
has also been used to predict strength development. Ono ( 0 7 ) described obtained from examinations of powder mounts. The values of four parameters, indicated in Table 4.1, were each estimated on a scale of 1 - 4 and the strength, R, in MPa, of a mortar a t 28 days then predicted using the equation:
The four parameters AS, AB, BS and BC were considered to be measures of heating rate, maximum temperature, time a t that temperature and cooling rate, respectively. As noted in the preceding section, the first of these assignments is questionable, but this does not necessarily invalidate the results obtained by the method, which was tested by routine application in many plants of a major cement company over a long period. Some of the other workers using the method have, in contrast, had little success with the method (e.g. Ref. S27), and the generality of the procedure, or of the coefficients, remains to be established. The constant term in the equation probably subsumes terms relating to composition and cement fineness, which would have to be taken into account if a wider range of samples was considered. Table 4.1
Parameters of Ono's method (07) for predicting cement strength
AS (alite size, pm) AB (alitc birefringence) BS (belite size, pm) BC (belite colour)
Hydraulic activity Value of parameter
15-20 0.008-0.010 2540
Clear Excellent 4
30-40 0.005-0.006 15-20
40-60 0.002-0.005 5-10
Good
Yellow Average
Amber Poor
3
2
I
20-30 0.006-0.007 20-25 Faint yellow
4.3 Scanning electron microscopy, X-ray diffraction and other techniques
4.3.1
Scanning electron microscopy
Scanning electron microscopy (SEM) of polished sections of clinkers provides information essentially similar to that given by light microscopy, with two important additions. First, phases can be analysed chemically. Second, the image can be stored electronically and subsequently processed or analysed in various ways. The specimen is polished and coated with an electrically conducting layer, normally 30 nm of carbon, but not etched. The most generally useful technique is backscattered electron imaging, using a detector designed to maximize contrast from compositional differences and
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Cement Chemistrv
to minimize that from topography, combined with provision for X-ray microanalysis. This should include facilities both for obtaining X-ray images giving semiquantitative information on the spacial distributions of individual elements, and for obtaining complete and quantitative chemical analyses in regions of micrometre dimensions. Techniques are described in standard texts (e.g. Ref. G32), and the results of analyses of individual clinker phases were considered in Chapter 1. The intensity of the electrons backscattered from a particular region of a specimen depends approximately on the mean atomic number of the material of which that region is composed; more precisely, it is represented by the backscattering coefficient, q, which may be calculated from the formulae:
where the summation is over elements and C, is the weight fractlon of an element having atomic number Z, (G32). This equation assumes the accelerating voltage to be 20 kV. Assuming the typical compos~tionsgiven in Table 1.2 for the individual phases, this gives values of 0.17 1 for alite, 0.167 for belite, 0.168 for cubic aluminate and 0.178 for ferrite. The differences between these values are such that the grey levels of the alite, belite and ferrite phases o n micrographs are distinguishable to the eye, but those of the belite and aluminate are not; however, the phase boundaries are normally visible, presumably due to slight topographical contrast or compositional irregularity. Fig. 4.3 shows backscattered electron images of a typical clinker and of a polymineralic cement grain present in a fresh paste (S28). The resolution available is limited by the size of the reglon from which the backscattered electrons are produced, which is somewhat below 1 pm in each direction. Much higher resolution is obtainable using the secondary electron Image, but this depends mainly on topographical contrast and thus gives, at best, poor definition of the phases. The resolution glven by the X-ray image IS poorer than that given by the backscattered electron image. Fig. 4.3 Backscattered electron images of polished sections of (A) a Portland cement clinker and (B) grains of a Portland cement in a fresh paste. In both sections, alite is the predominant clinker phase. in (A). the relatively large, darker areas are of belite, and the interstitial material consists of dendritic ferrite (light) in a matrix of aluminate (dark); cracks and pores (black) are also visible. In (B), the belite forms well-defined regions, which are rounded, striated and darker than the alite; the interstitial material, present, for example, in a vertical band left of centre within the larger grain, consists mainly of ferrite (light) and aluminate (dark). Scrivener and Pratt (S28).
1 )
1
Properties of Portland Clinker and Cement
107
108
Cement Chemistry
SEM with image analysis has been used in the study ofclinker and ground cement (L17.S29). Scrivener (S29) stored the backscattered electron image and X-ray images for several elements from the same area of a specimen. She did not find it possible to distinguish the clinker phases from the backscattered electron images alone, but by computer combination of data from these and the X-ray images, the silicate phases could be distinguished from the aluminate phases. The method was used to study the phase distributions within individual grains of cement and to compare the volume and surface distributions. For the cements studied, the large grains consisted wholly, or almost wholly, of belite. Almost all the remaining grains appeared to be polymineralic, even including those that were only a few micrometres in size (Fig. 4.3B), but grains smaller than about 0.5 pm were not easily studied and may have been monomineralic. Overall, fracture during grinding appeared to be predominantly through alite, and to avoid phase boundaries. The proportions of interstitial phases exposed on the surfaces of the grains were significantly greater than those by volume in the material. The volume fractions of silicate and interstitial phases were compared with those determined by QXDA or obtained by the Bogue calculation.. The application of image analysis to SEM is a rapidly developing technique, and further development may be expected. In addition to giving areas of phases, it can provide information on such matters as the distributions of shape, size and surroundings of particles and of phases and pores within them. It is, of course, essential to examine a sufficient number of samples and fields to ensure that the results are representative, but the labour and subjectivity associated with light microscopic studies of a comparable nature are largely eliminated.
4.3.2 X-ray diffraction The potential uses of X R D powder diffraction in the study of clinker or anhydrous cement include the qualitative and quantitative (QXDA) determination of phase composition, and the determination of polymorphic modification, state of crystallinity and other features of individual phases. In principle, information on compositions of phases is obtainable through cell parameters, but, due to the lack ofadequate reference data, X R D is generally less satisfactory for the clinker phases than X-ray microanalysis. Table 4.2 gives the pattern of a typical Portland cement, with indications of the assignments of peaks to phases. Alite is normally so much the predominant phase that its pattern tends to swamp those of the other phases. All the stronger peaks of belite are overlapped by ones of alite. Other difficulties in quantitative analysis are
Properties of Portland Clinker and Cement
109
caused by variability of patterns of phases due to compositional or polymorphic variation, and for the ferrite phase, peak broadening due to compositional zoningor imperfect crystallinity. Other problems in quantitative work are those of QXDA in general; e.g. the material must be ground sufficiently finely that primary extinction of the more intense peaks is minimized, and that adequate mixing with an internal standard is possible, if one is used, but not so strongly that crystallinity is reduced. Milling to below 5 p m in an agate ball mill with cyclohexane as grinding aid, followed by backloading into the sample holder of a diffractometer, has been found satisfactory for specimen preparation (G30). Table 4.2 20"
X R D powder pattern of a typical Portland cement (T5)
rl(nm)
I,,
Main phases
20"
rl(nm)
I,,,
Main phases
GYP Fe r Ali GYP Alu
Ali, Bel Ali Bel Ali Be1
Ali Be1 Fer Ali Bel
Ali, Bel Ali, Bcl Fer Bel Bel
Bel, Ali Ah, Bel GYP Ali Ali
Ali, Be1 Ali Fer Alu Ali, Fer
Bel, G y p Ali, Bel, Fer Ah, Bel Alu, Ali Fe r
Ali Ali Ali Ali Alu Ali
CuK, radiation. I,, = relat~vepeak height (B = broad). Phases are given in decreasing order of their contributions to each peak, ones making only minor contributions being omitted. Ali = alite, Bel = belite. Alu = aluminate, Fer = ferrite, Gyp = gypsum.
Struble (S30) summarized QXDA studies on clinkers and cements. Most investigators have used some form of the internal standard method ( K 10). Rutile (TiO,) has been widely used as the internal standard, but other
110
Cement Chemtstry
substances, such as silicon, KBr, CaF, and corundum, have also been employed; corundum has the advantage of giving no overlaps with important cement peaks up to 60" 20. The method in its conventional form. in which only a single peak each of the standard and the phase to be determined are used, is moderately effective for alite, using the peak at approximately 51" 20, assuming that a means o f accommodating the splitting of this peak is found. It is less satisfactory for the aluminate and ferrite, and useless for belite, because of the weakness of their peaks and interference from those of alite. With any form of the internal standard method, i t is necessary to enlploy reference standards that are as near as possible to the phases in the material as regards composition, polymorphism and degree of crystallinity. This implies both knowledge of the characteristics of the particular clinker under examination and the ability to prepare the necessary specimens, or alternatively to isolate them physically from the clinker itself. One solution ((330) has been to prepare reference patterns for a wide range of specimens of each phase and to use those that appear to correspond most closely to those present in the clinker under examination. Clinkers for which results had been obtained by point counting have also been used (K 1 I). Gutteridge (G30) described a method, based on earlier work by Berger et al. (B37), in which a least squares fit is obtained between a diffractonleter trace of the sample with internal standard and a simulated trace summing the contributions from the component phases. The trace is obtained over the range of 24-39" 20 by step counting at 0.05 degree intervals using CuKa, radiation, and stored in a computer. Auxiliary procedures were devised for dealing with background levels, adjusting the peak widths and, to some extent, peak positions in the reference patterns to match those in the clinker, and matching the observed and computed patterns. Minor phases, such as lime, periclase, calcium sulphate phases and mineral additions (Chapter 9), could be included in the analysis. This method is probably the most satisfactory yet described, but still depends on the choice of standards. Its power could probably be increased, at some expense of apparatus and computer time, by extending the range of 20 values used. Some workers have used similar methods, but matching observed and calculated intensities integrated over selected, wider ranges of 20 (KI 1). This could avoid problems arising from variations in the patterns of the component phases, though a t some loss of sensitivity. The Appendix gives calculated patterns based on the results of X-ray structure determinations for the clinker phases. For each pattern, a 'Reference Intensity Ratio' (H23) is included. This is the integrated intensity of the strongest individual reflection (which may be a component of an overlap) relative to that of the strongest peak of corundum in a 1 : 1 mixture by
Properties of Portland Clinker and Cement
11 1
weight. In principle, i t is possible by using this ratio to dispense with a reference standard. The internal standard used need not be corundum, provided that its own reference intensity ratio is known. This method has not been tested with clinkers, but has obvious possibilities. Given the necessary crystal structure data, it would permit the calculation and use in QXDA of patterns corresponding to any desired composition and polymorph for each phase.
4.3.3 Chemical or physical methods for separation of phases Partial separations of the clinker phases can be effected by chemical extraction methods, which may be combined with XRD to identify and determine minor constituents. Of several reagents that have been used to dissolve the silicate phases, the most successful has been a solution of salicylic acid in methanol (SAM), introduced by Takashima (T10). This reagent dissolves free lime, alite and belite, at rates decreasing in that order (H24). By adjusting its composition, it is possible either to remove much of the alite, so facilitating investigation of the belite, or all of the alite and belite, facilitating that of the remaining phases. The procedure is as follows (H24). The clinker is ground in propan-2-01 to an average particle size of 5 pm. After evaporating off the propan-2-01, the sample (5 g) is treated for 2 h at room temperature with the SAM reagent (300 ml), with constant stirring. The solution, which is now red, is filtered on a No.4 sintered glass filter. and the residue washed with methanol and dried at 90•‹C.If the residue is to be examined by QXDA, the internal standard (e.g. rutile) may be mixed with the sample before the extraction. For removal of alite and belite, the salicylic acid content of the reagent is 20g per 300ml. For complete removal of the alite, leaving the maximum quantity of belite undissolved, it is 10-15 g, depending on the alite content; five times the weight of C,S in the sample, as given by the Bogue calculation, was found to be satisfactory. A solution of maleic acid in methanol has also been used to dissolve the silicate phases (TI 1). This is quicker than the SAM method, but if the methanol contains water, ettringite may be formed and water-soluble phases, such as K,SO,, lost, while if it is anhydrous, gelling occurs; the SAM method is thus preferable (S31). Extraction of the silicate phases by the trimethylsilylation method of Tamas et al. (T12) (Section 5.3.2) also leaves a residue containing the aluminate and ferrite phases. Gutteridge (G33) found that a solution of sucrose in aqueous KOH (KOSH reagent) dissolves the aluminate and ferrite, leaving the silicates and minor phases. The clinker or cement (10 g) is reduced to below 5 pm by
1 12
Cement Chemistry
grinding in cyclohexane (20ml) for 40min in an agate ball mill. After evaporating off the cyclohexane in a stream of N,, the sample (9 g) is added to the KOSH solution (30 g of KOH and 30 g of sucrose in 300 ml of water) at 9 5 ~ Cand stirred for 1 min. The solution is then filtered and the residue washed in water (50 ml) followed by methanol (I00 ml) and dried at 60•‹C. By using a combination of the SAM and KOSH procedures, i t is possible to produce a residue consisting largely of belite. Various solvents may be used to dissolve free lime, which can be determined by titrating the resulting solution with a suitable acid. In one procedure (J7), the clinker or cement (I .0 g), ground to about 300 mZkg-' (Blaine), is treated with anhydrous ethylene glycol (50 ml) at 80-100•‹C for 5 min, using a magnetic stirrer. The solution is then filtered under suction and the residue washed at least twice with a further 10-15 ml of hot ethylene glycol. The filtrate and washings are diluted with 25 ml of water and titrated to phenolphthalein with 0.05 M HC1. Other procedures employ mixtures of ethanol and glycerol (L18) or of acetoacetic ester and butan-2-01 (Franke solvent) (F8). If properly used, all give similar results. They also dissolve CH, and have been used to determine this phase in hydrated cements, but their use for this purpose is questionable as they also appear to attack other hydration products. Physical methods are of limited effectiveness for separating the phases in cement clinker because of the intimate scale on which the latter are mixed; however, Yamaguchi and Takagi (YI) had some success with the use of dense liquids. Some concentration of the ferrite phase can be effected by magnetic separation (M35,Y 1).
4.3.4
Other methods
Bensted (B38) described the use of TG in the study of clinkers or unhydrated cements. At 4"Cmin-' in Nz with cements, losses normally occur at 100-200•‹C from gypsum or hemihydrate, 400-500•‹C from CH and 500800•‹Cfrom CaCO,. If a normal, open sample cup is used, the losses from conversion of gypsum to hemihydrate and from dehydration of hemihydrate are poorly resolved, making interpretation difficult. Better separation can be achieved by using a sample cup with a top that is closed except for a narrow exlt (S32). Syngenite, if present, decomposes at 250-300•‹C. Small amounts of CH can occur due to hydration, and of CaCO, due to carbonation during milling or storage, impurity in the gypsum, or, where specification allows it, deliberate addition. The increasing tendency to allow additions of sub- 4 stances other than calcium sulphate may lead to the presence of other phases ! I
Properties of Portland Clinker and Cement
11 3
detectable by TG. A weight gain at 300400•‹Cin air that does not occur in N, normally indicates that the clinker was made under reducing conditions, but could also conceivably be due to the presence of an excessive quantity of metallic iron incorporated during milling. In this latter case, it would, of course, not be shown by the clinker. Applications of IR and Raman spectroscopy to the study of clinkers and unhydrated cements have been reviewed (B39,B40). The laser Raman microprobe, with which regions of micrometre dimensions on a polished surface may be examined, has been used to investigate structure and crystallinity, especially of the alite and belite (C19). Spectroscopic methods for studying the surfxe structures and compositions of cements are considered in Section 5.6.2.
4.4
4.4.1
Quantitative phase compositions General
The methods that have proved effective for quantitative determination of phases in clinkers are based on light microscopy (Section 4.2.1), X-ray diffraction (Section 4.3.2) and calculation from the bulk chemical analysis. The last two of these are applicable also to cements. SEM with image analysis (Section 4.3.1) shows promise, but other methods that have been investigated, such as IR spectroscopy, appear to have little potential. Sections 4.4.2 to 4.4.5 describe a method of calculation from the bulk analysis and Section 4.4.6 compares the results of the various methods.
4.4.2 Calculation of quantitative phase composition from bulk analysis As will be seen in Section 4.4.6, there is wide agreement that the Bogue calculation gives seriously incorrect results, especially for alite. It has often been stated that the errors arise because equilibrium is not maintained during cooling, but this is only partly correct. Even if equilibrium was maintained, all the major phases would contain significant proportions of substituent ions. The calculation gives incorrect results because the phases do not have the compositions assumed for them, and the absence of equilibrium conditions during cooling merely alters the compositional differences from the pure compounds. For a calculation of the quantitative phase composition of a clinker from the bulk chemical analysis to give correct results, the following conditions are necessary and sufficient:
1 14
Cement Chemistry
( I ) The analysis, including any auxiliary data such as the content of free lime, must be free from errors. (2) The analysis must be corrected for any of the four major components not present in the four major phases. This is routinely done for free lime, but CaO may also be present in sulphate phases, and, assuming modern analytical methods (e.g. X R F ) to have been used, some of the SiO, and A1,0, will normally be present in an acid-insoluble residue. (3) The contents of the four major components in each of the four major phases must be known. If necessary, weighted mean values must be used to allow for variable composition, arising, for example, from zoning in alite or ferrite, or from the presence of both cubic and orthorhombic aluminate. Belite presents a special problem because of the probable occurrence of material exsolved between the lamellae. In the present state of knowledge, use of an average composition including this material, such as is given by X-ray microanalysis, is the most practical solution.
The Bogue calculation is a solution of four linear simultaneous equations for four unknowns. Following Yamaguchi and Takagi (Y 1 ) and Harrisson cr 01. (H3), one may modify it by using the best available estimates of the compositions of the four phases instead of those of the pure compounds. If these compositions were free from error, and the other conditions noted above were satisfied, this procedure would necessarily give correct results. For modern, rapidly cooled clinkers of ordinary Portland cements, the compositions of the phases do not seem to vary excessively, apart from some variation in the contents of MgO, CaO and Fe20, in alite (Chapter 1). By using typical compositions, such as those given in Table 1.2, one may expect to obtain results that are substantially more accurate than those given by the Bogue calculation. The following procedure (TI 3) is based on this approach, and in the form described in Sections 4.4.3 and 4.4.4 applies to most modern clinkers. Limitations and modifications for cements or other types of clinkers are noted in Section 4.4.5. It is illustrated by an example, in which the following analysis is assumed: Na,O MgO A1,0, SiO, P 2 0 , SO, K 2 0 CaO TiO, Mn,O, Fe20, Loss Total 0.22 1.2 5.7 21.7 0.20 0.37 0.45 65.9 0.29 0.06 3.2 0.6 99.9 Free lime, 0.6%; insoluble residue, 0.3%; main analysis by XRF.
4.4.3 Estimation of sulphate phases The procedure given below provides an estimate of the amounts of K20, Na,O, CaO and SO, present in sulphate phases. It is based on the results of Pollitt and Brown (P2) (Section 3.5.6), and more specifically on the curves in
11 6
Cement Chemistry
The solution to these equations, obtained by inverting the matrix A, is
The calculated percentages of alite, belite, aluminate and ferrite are given by .Y,-s,, respectively. The Bogue calculation gives 60.3%, 16.1%, 9.4% and 9.7%, assuming that the SiO, and A1203 contents have been corrected for insoluble residue. Any standard method of matrix inversion, such as the Gauss-Jordan method (N13), may be used to solve the equations. The coefficients in equations 4.114.14 may be used without serious error for most ordinary Portland cement clinkers in which the alite composition is not too different from that assumed here. As a byproduct of the calculation described in this section, and using the full compositions of the phases given in Table 1.2, one may calculate a mass balance table (Table 4.3) showing the distributions of all the oxide components among the phases. 4.4.5
Limitations and modifications of the calculation of phase composition
The procedure described above should give satisfactory results for most modern ordinary Portland cement clinkers, but the following points should be noted: (1) If the aluminate phase is orthorhombic, the appropriate composition (Table 1.2) should be substituted in the matrix A . If both cubic and orthorhombic forms are present, intermediate values may be used. (2) If the result for aluminate is negative, this phase is absent and it composition should be replaced in the matrix A by that of the low aluminium ferrite phase given in Table 1.2. From the composition and proportions found for the two ferrite components, the conten and composition of the ferrite may be estimated. Similarly, if th result for ferrite is negative, the ferrite composition is replaced by tha of low-iron aluminate. These conditions will occur at ARs below abou 1.0 and above about 4.0, respectively. The data for the low-aluminiu
Table 4.3 Distribution of oxide components among phases in a typical Portland cement clinker, calculated from the bulk chemical analysis Na,O
MgO
A1,0,
SiO,
P,O,
SO,
K,O
CaO
TiO,
Mn,O,
Fe,O,
Total
Alite Belite Aluminate Ferrite Free lime Periclase
K 3NS4 C2KS3
Insol. res. Other Total
-
Results given as percentages on the ignited weight. Periclase calculated by difference. 'Other' comprises errors arising from rounding, incorrect assumptions regarding compositions of phases, and contributions from any phases not included in the calculation.
1 18
Cement Chemistry
and low-iron aluminate in Table 1.2 are tentative, and no high accuracy may be expected. (3) The results will be less accurate for slowly cooled clinkers, as the compositions of the ferrite and possibly also the aluminate phases may differ significantly from those assumed here. At present, there are not enough data to deal with this problem. The method is not applicable without major modification to clinkers made under reducing conditions. It is doubtful whether the procedure is applicable to white cements, both for this reason and because they may contain glass. The calculation of phase composition from bulk analysis for cements presents no special difficulties if the content and composition of the gypsum and any other admixture are known, as the analysis can then readily be corrected to give the composition of the clinker. If they are not known, an assumption must be made as to the amount of SO, in the clinker. so that the contribution of the gypsum to the CaO content of the cement may be estimated. Further assumptions and corrections may have to be made to allow for impurities in the gypsum or other additives (e.g. calcite).
4.4.6 Comparison of results of different methods From the results of a study in which six clinkers were examined in a number of different laboratories, Aldridge (A6) concluded that light n~icroscopy gave accurate results for the quantitative determination of phases in clinkers, assuming an experienced operator and adequate sampling. Results obtained using QXDA varied widely between laboratories, and it was concluded that, as carried out in most of them, the method was of low accuracy. One laboratory, however, obtained results that agreed well with those given by microscopy, and which were considered to be accurate. In general, the contents of alite found by microscopy were higher than those given by the Bogue calculation. The results of another cooperative study, in which microscopy and extraction methods were used, showed that the Bogue calculation alw gives low results for alite and generally gives high ones for belite wh compared with microscopy (C18). For the aluminate and ferrite phase agreement was reasonably good. The total contents of silicates, determined by chemical extraction, agreed reasonably well with those found by microscopy. Several early studies using QXDA gave results in fair agreement wi those of the Bogue calculation, but in the light of the above and other rece evidence are of doubtful validity. Odler et a/. (09) found that alite conten
\
Properties of Portland Clinker and Cement
119
found either by microscopy or by QXDA were always greater than those given by the Bogue calculation. Gutteridge (G30) reached the same conclusion for QXDA. and also noted that the total amounts of the crystalline ~ h a s e sdid not vary significantly from 100%. Kristmann (K 12) concluded from a study of 39 clinkers that alite and belite were determined more satisfactorily by microscopy than by QXDA, but that aluminate and ferrite, due to their close admixture, were better determined by QXDA o r by determining the total content of interstitial material by microscopy and the relative amounts of its constituent phases by Q X D A of a residue from SAM extraction (Section 4.3.3). His results indicated that the values given by the Bogue calculation are low for alite and aluminate, high for belite, and reasonably satisfactory for ferrite. The mean discrepancy for alite was 8%. The method for calculating phase composition from bulk analysis described in Sections 4.4.2-4.4.4 was tested using Kristmann's data (T13). Agreement was substantially better than with the Bogue calculation for alite, belite and aluminate, and about as good for ferrite. For alite and belite, there was no significant bias in either direction. For aluminate, there was a possible tendency to give high results; for ferrite, it was not possible to tell whether there was bias, due to inadequacies in the experimental data. Good agreement was also obtained in tests on a wide range of cements, using experimental data obtained by Gutteridge (G34) using QXDA. Taken as a whole, the evidence indicates that all four major phases can be satisfactorily determined by QXDA if an adequate experimental procedure is employed (e.g. Ref. G30) o r by calculation. Alite, belite and the total content of interstitial material can also be determined by microscopy. With any of these methods, assuming the best available procedures to be used, the absolute accuracy is probably 2-5% for alite and belite, and 1-2% for aluminate and ferrite.
4.5 R e a c t i v i t i e s of c l i n k e r p h a s e s 4.5.1
Effect of major compositional variation
The ability of a substance to act as a hydraulic cement depends o n two groups of factors. First, it must react with water to a sufficient extent and at a sufficient rate; y-C,S, which is virtually inert at ordinary temperatures, has no cementing ability. Second, assuming that a n appropriate ratio of water to cement is used, the reaction must yield solid products of very low solubility and with a microstructure that gives rise to the requisite mechanical strength, volume stability, and other necessary properties. C,A reacts rapidly and completely with water, but the products that are formed when
120
Cement Chemistry
no other substances are present d o not meet these criteria, and its ability to act as a hydraulic cement is very poor. C3S, in contrast, satisfies both sets of conditions, and is a good hydraulic cement. In this section, we consider only the factors that control reactivity. Any understanding of them must be based on a knowledge of the mechanisms of the reactions with water. It is highly probably that these are in all cases ones in which an essential step is transfer of protons to the solid form from the water, which is thus acting as a B r ~ n s t e dacid (D9.B41). As an initial approximation. one may consider the problem purely in terms of the idealized crystal structures. The reactivities of the oxygen atoms towards attack by protons depend on their basicities, i.e. the magnitudes of the negative charges localized on them. Any structural feature that draws electrons away from the oxygen atoms renders them less reactive, the basicity thus depending on the electronegativities of the atoms with which they are associated. In this respect, the nearest neighbours are the most important, but the effects of atoms further away are far from negligible. This hypothesis leads to the following predictions, all of which agree with the facts: (1) Oxygen atoms attached only to atoms of a single element will be less reactive as the electronegativity of that element increases. Thus, C3S and lime both contain oxygen atoms linked only to calcium. and are more reactive than B-C2S, in which all the oxygen atoms are also linked to silicon, while a-A1203,in which all are linked to aluminium, is inert. However, the reactivity of lime decreases if it is strongly burned; this is due to effects discussed later. (2) Oxygen atoms forming parts of silicate, aluminate or other anionic groups will be less reactive as the degree of condensation with other such groups increases. p-C2S, in which the tetrahedra d o not share oxygen atoms, is more reactive than any of the C3S, or CS polymorphs, in which they do. (3) Oxygen atoms forming parts of anionic groups similar as regards degree of condensation will be less reactive as the central atoms in the groups become more electronegative. In contrast to the C3S, and CS polymorphs, CA, C,,A, and C3A, in which the AIO, tetrahedra share corners, are all highly reactive. On the other hand, C3P, which contains isolated tetrahedra, is unreactive. Other hypotheses based on considerations of crystal structure have been proposed, and in varying degrees may account for second-order effects. Thus, Jeffery (JI) suggested that irregular coordination of calcium was responsible for the high reactivity of C,S as compared w ~ t hy-C2S; however, ; lime reacts with water, but the coordination of its calcium atoms is highly '
Properties of Portland Clinker and Cement
121
symmetrical. Jost and Ziemer (J8), who noted the weaknesses in this and other earlier hypotheses, considered that high reactivity was associated with the presence of face-sharing CaO polyhedra, but lime contains no such groupings, yet can be highly reactive.
4.5.2
Effects of ionic substitutions, defects and variation in polyrnorph
The considerations discussed in the preceding section suffice only to explain the effects on reactivity of major variations in composition. They do not, in general, explain the effects of variations in polymorph or introduction of foreign ions. These effects are often complex, and while in some cases the results appear clearcut, there are many others in which they present a confusing and sometimes apparently contradictory picture. The probable reason is that many variables are involved which are difficult or impossible to control independently. These include, besides chemical composition and polymorphic change: concentrations and types of defects; particle size distribution; textural features such as crystallite size and morphology, mechanical stress, presence of microcracks and the mono- or polycrystalline nature of the grains; and influences of reaction products, whether solid or in solution. In a cement as opposed to a pure phase, a foreign ion may not only substitute in a particular phase, but also influence the microstructure of the material in ways unconnected with that substitution, e.g. by altering the physical properties of the high-temperature liquid present during its formation. For both microstructural and chemical reasons, the reactivity of a given phase in a cement may not be the same as that observed when i t is alone. Defects play a particularly important role. Sakurai et al. (S33) noted that C,S, alite, C,F and C,AF could all take up C r 2 0 3 and that this greatly accelerated reaction at early ages. Electron microscopic studies showed that reaction began at grain boundaries and at points of emergence of screw dislocations, the concentration of which was greatly increased by C r 2 0 , substitution. It was also shown that the substituted materials were semiconductors, and a mechanism of attack based on electron transfer processes was suggested. Fierens and co-workers (F9,FlO) also found that the influence of foreign ions on the reactivity of C3S was due to the presence of defects, which could be studied using thermoluminescence, and which could also be introduced by suitable heat treatment. The complex relations between content of foreign ions, polymorphism, defects and reactivity are well illustrated by the results of Boikova and coworkers (B42,B4) on ZnO-substituted C,S. With increasing ZnO content. triclinic, monoclinic and rhombohedra1 polymorphs were successively stabi-
122
Cement Chemistry
lized. Curves in which the extents of reaction after given times were plotted against ZnO content showed maxima corresponding approximately to the T2-M, and M,-R transitions, with intermediate minima. As in the work mentioned above, it was shown that these compositions were also ones in which the numbers of defects of a particular type were at n marimurn. Reactivity thus depends not so much on the amount of substitucnt or- nature of the polymorph, as on the types and concentrations of defects. Studies on the comparative reactivities of P-, a'- and a-C,S. reviewed by Skalny and Young (S34), have given contradictory results. As these authors conclude, reactivity probably depends on specimen-specific factors other than the nature of the polymorph, and in the lower temperature pol!morphs may be affected by the exsolution of impurities. This'latter hypothesis receives strong support from subsequent observations that attack begins at exsolution lamellae and grain boundaries, and that these may contain phases, such as C,A, that are much more reactive than the C?S (CI). Without these lamellae, p-C2S might be much less reactive than i t is. There is wide agreement that substitution of alkali metal ions retards the early reaction of the aluminate phase, which is thus less for the orthorhombic than for the cubic polymorphs (S35,B43,R13). The effect has been attributed to structural differences, but the early reaction of pure C,A is also retarded by adding NaOH to the solution, and the OH- ion concentration in the solution may be the determining factor (S35). The reaction of C,A is also retarded by iron substitution and by close admixture with ferrite phase; formation of a surface layer of reaction products may be a determining factor, at least in later stages of reaction, and the retarding effect of such a layer may be greater if it contains Fe3+ (B44). series have The reactivities of ferrite phases in the pure Ca,(Al,Fe, been widely found to increase with AI/Fe ratio, and material of composition similar to that in clinkers is more reactive than either C,AF or C,A,F (e.g. Refs. DIO,B43 and B44). Anomalies in the relation between rate and AlIFe ratio found by some workers can possibly be attributed to zoning or other variations in crystallinity. As with the aluminate phase, the decrease in reactivity with iron content is possibly explainable by differences in the properties of a retarding layer of product. The relatively high reactivity of the clinker material may be related to its semiconducting properties (M 18) and to disorder arising from the complex ionic substitutions (B44).
-
Hydration of the calcium silicate phases 5.1 Introduction 5.1 .I
Definitions and general points
In cement chemistry, the term 'hydration' denotes the totality of the changes that occur when an anhydrous cement, or one of its constituent phases, is m~xedwith water. The chemical reactions taking place are generally more complex than simple conversions of anhydrous compounds into the corresponding hydrates. A mixture of cement and water in such proportions that setting and hardening occur is called a paste, the meaning of this term being extended to include the hardened material. The watertcement (w/c) or water/ solid (w/s) ratio refers to proportions by weight; for a paste, it is typically 0.3-0.6. Setting is stiffening without significant development of compressive strength, and typically occurs within a few hours. Hardening is significant development of compressive strength, and is normally a slower process. Curing means storage under conditions such that hydration occurs; conditions commonly employed in laboratory studies include storage in moist air initially and in water after the first 24 h, storage in air of 100% relative humidity and, less favourable for reaction, storage in a sealed container. Because Portland cement is a relatively complex mixture, many studies having the aim of elucidating its hydration chemistry have been made on its constituent phases. For a given particle size distribution and w s ratio, tricalcium silicate or alite sets and hardens in a manner similar to that of a typical Portland cement. Using XRD and other methods, it may be shown that about 70% of the C,S typically reacts in 28 days and virtually all in I year, and that the products are calcium hydroxide (CH) and a nearly amorphous calcium silicate hydrate having the properties of a rigid gel. PC,S behaves similarly, but much less CH is formed and reaction is dower, about 30% typically reacting in 28 days and gooh in I year. In both cases, reaction rates depend on particle size distribution and other factors. Development of compressive strength runs roughly parallel to the course of the chemical reactions, and the strengths at 1 year are comparable to those of Portland cements of the same w/s ratio and cured under the same conditions. The calcium silicate hydrate formed on paste hydration of C,S or P-C,S is a particular variety of C-S-H, which is a generic name for any amorphous
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or poorly crystalline calciuni silicate hydrate. The dashcs indicate that no particular composition is implied, and are necessary because CSH in cement chemical nonienclature denotes material specifically of composition CaO.SiO;H,O. The term 'C--S-H gel' is sometimes used to distinguish rhe material formed in cement. C,S or P-C,S pastes from other varieties of C-S-H. This chapter deals primarily with the reactions and products of hydration of C,S and P-C,S in pastes at ordinary temperatures. Except where stated, a temperature of 15-25'C is assumed. Background information has been obtained from studies of C,S or I)-C,S hydration at higher w/s ratios. i.e. in aqueous suspensions, or on C-S--H prepared in other ways, and the results of such studies are also considered.
5.1.2 Experimental considerations In all laboratory studies on the hydration of cements or of their constituent phases, or of C-S-H or other hydrated phases formed by other methods, rigorous exclusion of atmospheric CO, is essential. Contamination by atmospheric CO, ('carbonation') is apt to be much more serious with small laboratory samples than with practical concrete mixes of snialler surface to volume ratio. All operations, such as mixing. curing or drying of pastes, or preparation, filtration or drying of solid phases in suspensions, must be carried out in a C0,-free atmosphere, either in a dry box or in apparatus otherwise designed to isolate the materials from the atmosphere. Wet or moist niaterials are especially susceptible to attack, but dry ones are not immune. Water must be deionized and freshly boiled. Prolonged contact with glass must also be avoided; wet curing may be carried out in the presence of a small excess of water in a sealed plastic container. Despite all precautions, hydrated materials will inevitably contain some CO,, analysis for which will often be required. Jones's (J9) method, based on decomposition with dilute acid and absorption of the CO, in Ba(OH), solution, which is then back-titrated with HCI, is simple and accurate. TG has often been used to determine CO,, but is not satisfactory because the loss at 700900•‹C is not wholly due to CO,. Absence of calcite peaks from an XRD pattern is not a proof that CO, is absent, as CO, can occur in other forms, e.g. as vaterite (p-CaCO,), in calcium aluminate hydrate phases and possibly within the C-S-H structure. Especially with relatively young pastes. the problem often arises of removing excess water after a specified time of curing to stop the hydration reactions and to render the material less susceptible to carbonation. For this' purpose, soaking or grinding in polar organic liquids, such as methanol,
Hydration of the Calc~urnSilicate Phases
125
propan-2-01 or acetone, has often been employed, but should be used only in situations where it will not invalidate the results of subsequent tests. Organic substances can be completely removed from C,S pastes only b!, drastic procedures that seriously alter the material. Their presence is especially undesirable if any form of thermal analysis (e.g. TG or DTA) is to be carried out, since on heating they react with C-S-H to give CO, (Dll.Tl4). Acetone has especially complex effects, since in the alkaline medium i t rapidly undergoes aldol condensations at room temperature. giving niesityl oxide. phorone and other relatively involatile products (T14). Some workers consider that primary aliphatic alcohols alter the pore structures of C,S or cement pastes or of synthetic C-S-H preparations (Section 8.3.2) and i t has also been claimed that methanol reacts with CH at ordinary temperatures, giving calcium methoxide (B45). In choosing a procedure to remove excess water, it is necessary L O consider the likely effects on the tests subsequently to be performed. Retention of organic substances is not the only possible complication; undul!. drastic drying procedures, e.g. heating at 105•‹Cor equilibration to constant weight in atmospheres of low relative humidity, such as 'D-drying' (Section 5.2.2). partially dehydrate the C-S-H and, with cement pastes. can also decompose crystalline hydrated calcium aluminate or aluminate sulphate phases. For some procedures, such as XRD, the presence of organic material may not matter, but even here, the possibility of decomposing or altering hydrated aluminate phases must be considered. For electron microscopy, including the preparation of polished sections for SEM, freeze drying has been found satisfactory (J10). For many purposes, including thermal analysis above about 150•‹C,pumping with a rotary pump for I h is suitable. The drying of samples for studies on pore structure is considered in Section 8.3.3. All analytical data on hydrated cements or cement constituents should normally be referred to the ignited weight. The original weight includes an arbitrary amount of water, and results referred to it, or to the weight after some arbitrarily chosen drying procedure (e.g. to constant weight at 105 C), are often impossible to interpret in detail.
5.1.3 Calcium hydroxide CH has a layer structure (Fig. 5.1) (B46,P18). The calcium atoms are octahedrally, and the oxygen atoms tetrahedrally, coordinated. The interlayer forces are weak, with negligible hydrogen bonding, thus giving good (0001) cleavage. The unit cell is hexagonal, with u = 0.3593 nm, c = 0.4909nm, space group ~ j m l , Z = 1, D, = 2242kgm-3. The optical properties are: uniaxial -, w = 1.573, E = 1.545 (S5). Calcium hydroxide occurs as a natural mineral, known as portlandite.
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Plan
Fig. 5.1 Structure of a single layer of Ca(OH),. Small, full circles represent calcium atoms. large open or shaded circles oxygen atoms; a hydrogen atom. not shown, completes the tetrahedral coordination of each oxygen atom. Taylor (T15).
Under ideal conditions of crystallization, C H forms hexagonal plates. Admixtures greatly affect the morphology, and especially the c : rr aspect ratio (B47). Euhedral crystals are observed in young pastes and in pores of older pastes, but as hydration proceeds, the main deposits of C H become massive and of indeterminate shape, though the good cleavage persists. Several studies o n the C H of cement pastes by light microscopy or X-ray microanalysis have indicated the presence of small amounts of SiO, and other components, but it is uncertain whether these occur as ionic substituents in the C H structure or in inclusions (B48). There does not appear to be any convincing experimental evidence for claims that some of the C H in pastes is amorphous. A T E M study of ion-thinned sections of a C,S paste of normal w/s ratio showed the C H to occur as large. imperfect crystals; no finely dispersed, microcrystalline material was observed, though Some crystals were only tens of nanometres thick (G35,G36); cryptocrystalline CH was found only in pastes of abnormally low w/s ratio (G37). A reported occurrence of amorphous C H in a Renaissance lime plaster (M36) was disproved (N14), but a natural occurrence of amorphous C H has been described ( N 15). It has been reported that the 0001 peak of CH is anomalously strong relative to the 10il peak in XRD powder patterns of calcium silicate or
'
Hvdration of the Calcium Silicate Phases
127
cement pastes obtained using a diffractometer, to an extent that increases with the w/s ratio and decreases with age (G38,28). Grudemo ((338) attributed the effect to overlap between 0001 and a peak from C-S-H. Although care was taken to minimize preferred orientation in these studies, judgement on the results must be reserved. Because of the good cleavage. preferred orientation occurs very readily, and if a diffractometer is used. greatly increases the relative intensity of 0001. Such orientation could be produced in various ways, e.g. on sawing a hardened paste to prepare a flat surface. Stacking faults can also affect the pattern (S36). The dissolution of C H in water is exothermic (AH = - 13.8 kJ n ~ o l -' at 25•‹C)(G39), and the solubility therefore decreases with temperature (Table 5.1). The data given are for large crystals; higher apparent solubilities. up to at least 1.5 g CaO I - ' at 25"C, readily occur with finely crystalline material. Table 5.1
"
Solubility of calcium hydroxide ( S ;g CaO 1 - ') (Bassett. B49)
Interpolated values.
-
I -
Temperature
(OC)
Fig. 5.2 Thermogravimetric curve of Ca(OH), (0.2% CO,); 50 mg sample heated at 10degC min-' in dry, C0,-free N, flowing at 15 ml min-'. Taylor (T5).
Halstead and Moore (H25) reported thermal decomposition equilibria. The decomposition pressure reaches 101 kPa (1 atm) a t 512•‹Cand the mean enthalpy of decomposition a t 300-510•‹C is 102.9 kJ mol- '. When heated in
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Cement Chemistry 1
a vacuum or in a stream of dry N,, CH begins to decompose at an easily
detectable rate at about 370•‹C.Under typical conditions for TG, deconiposition is 980h complete at the end of the step at 580•‹C(Fig. 5.2). An earlier conclusion that only about has decon~posedat this point (T16) is incorrect. The finely divided CH available as a laboratory or analytical reagent normally contains several per cent of CaCO,, and the curve in Fig. 5.2 is for coarsely crushed large crystals containing 0.2% CO,.
5.2 Compositions of C,S and P-C,S pastes 5.2.1
Calcium hydroxide content, thermal analysis and indirect determination of the Ca/Si ratio of the C-S-H
The Ca/Si ratio of the C-S-H in 21 fully reacted C,S or 0-C,S paste may be calculated if the contents of CH and CO, are known, though an assumption must be made as to whether the CO, occurs in the C-S-H or as CaCO,. If the content of unreacted C,S or P-C,S is also known, the method can be extended to incompletely reacted pastes, though small errors in the contents of CH or unreacted starting material greatly affect the result at low degrees of reaction. Quantitative XRD is probably the only method presently available for determining unreacted C,S or 0-C,S; the precision is probablj not better than 2 3% (010). Methods used to determine CH have included TG, DTG, semi-isothermal DTG, thermal evolved gas analysis, DTA, differential scanning calorin~etry(DSC). QXDA, IR spectroscopy, image analysis of backscattered electron images and extraction methods using either aqueous media or organic solvents similar to those used to determine free lime in anhydrous cements. Some workers have found significant differences between the results of thermal methods and QXDA, the latter usually giving lower results for the CH content (M37.OIO,B50,RI4.D12), but others have found no differences outside experimental error (L19,T17). There is broad agreement that extraction methods give higher results than either thermal methods or QXDA (010,M37,B50,R14). It has been suggested that QXDA gives low results due to the presence of amorphous CH (R14). Thermal methods have also been considered to give low results for the same reason (OIO), or because of CH adsorbed on the C-S-H (BSO) or present as interlayer material (S37). Sources of error in CH determination by TG have been discussed (T16). AS noted earlier, major errors can arise from the use of thermal methods on material that has been in contact with organic liquids (T14). Preliminary results from image analysis did not correlate well with those from T G (S38): In the writer's view, adsorbed or interlayer material is not CH, but part of
h
Hydration o f the Calcium Silicate Phases
129
the C-S-H. The absence of evidence for amorphous CH, or, except at very low w/s ratios, of cryptocrystalline CH, was noted earlier (Section 5.1.3). The higher results given by extraction methods are probably attributable to prtial decomposition of the C-S-H. Of all the methods, TG and DTG are pobably the least open to criticism, though precision and accuracy are limited by the difficulty of distinguishing sharply between the effects due to CH and the other phases present. DTA is somewhat suspect, because the peak area for CH is reported to depend markedly on particle size (W 17). Fig. 5.3 shows the TG and DTA curves of a fully reacted C3S paste, obtained under the same conditions as the TG curve of CH in Fig. 5.2. The paste had been prepared at w/s = 0.45 and stored wet, in a sealed container, for 25 years at 25•‹C; it contained 0.8% of CO,, referred to the ignited weight. The height of the CH step (mean from seven determinations and estimated as shown in the inset) was 8.9%, referred to the ignited weight. Assuming that this step represents the decomposition of 98% of the CH and that the CO, occurs as CaCO,, there is 1.15 mol CH and 0.04 mol CaCO, per mole C,S, and the Ca/Si ratio of the C-S-H is 1.8 1.
DTA-
~
Temperature
~
( O C )
Fig. 5.3 Thermogravimetric and differential thermal analysis curves for a fully reacted C,S paste; experimental conditions as for Fig. 5.2. After Taylor and Turner (T14).
This result is similar to those of several other recent investigations, which have indicated Ca/Si ratios of 1.99 using QXDA or 1.60 using an extraction method (OlO), 1.7 $ 0.1 (L19) or 1.8 (implicitly) (P19) for mature pastes of C,S or alite and of around 1.8 (K13,Fll) or 2.0 (S18) for ones of P-C,S.
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There is evidence of a lower Ca/Si ratio during the first few hours of reaction (010). but with that reservation the more recent work (OIO.L19,P19) does not support the conclusion of some earlier workers that the ratio changes markedly during the course of the reaction.
5.2.2 Water content of the C-S-H
Evidence considered in Sections 5.3.1 and 8.3 indicates that the C-S-H gel of calcium silicate or cement pastes has a layer structure. and that. together with a pore solution, i t forms a rigid gel in which the pores range in size from macroscopic to enlarged interlayer spaces of nanometre dimensions. One can therefore define a water content only in relation to a specified drying condition. Three such conditions will be considered. The most highly hydrated state is that existing in a saturated paste, i.e. one in which the pores are totally filled with water. According to the theory of Powers and Brownyard (P20) (Section 8.2. I), the C-S-H in a cement paste can form only if sufficient space is available to permit i t to be accompanied by a defined proportion of pore space. There is thus a certain wic ratio below which complete hydration is impossible, and for which a mature paste consists entirely of hydration product, including this essential pore space. The total water content of such a paste in the saturated condition is typically 4244%. referred to the ignited weight. For nearly saturated C,S pastes, Young and Hansen (Y5) concluded that the composition of the C-S-H was approximately 1.7Ca0.Si0,.4H20. Taking into account the water present in the CH, this corresponds to a w/c ratio of 0.42, which is similar to the values found for cement. Chemically bound water is most reasonably defined as including that present in interlayer spaces, or more firmly bound, but not that present in pores larger than interlayer spaces. As will be seen in Chapter 8, the distinction between interlayer space and micropores is not sharp: water adsorbed on surfaces of pores further blurs the definition. From the experimental standpoint, the determination is complicated by the fact that the amount of water retained at a given RH depends on the previous drying history of the sample and on the rate at which water is removed. An approximate estimate is obtained by equilibrating a sample, not dried below saturation, with an atmosphere of 1 1 % RH (F12,F13,F14). Saturated aqueous LiC1.H20 gives the required RH (partial pressure of water vapour = 2.7 torr at 25•‹C). To achieve apparent equilibrium in a reasonable time (several days), the sample must be crushed and the system evacuated; the salt solution should be stirred, at least intermittently. Young and Hansen (Y5) found the composition of the C-S-H in C,S paste thus
Hydration of the Calcium Silicate Phases
131
equilibrated to be 1 .7Ca0.Si02.2.1 H,O. For C,S hydrated in suspension, which probably gives a similar product, Feldman and Ramachandran (F14) found the C-S-H composition to be 1 .64Ca0.Si02.1.96H,O. As noted earlier, treatment of C3S pastes with methanol leavcs some of the latter strongly sorbed. The methanol is only removable on heating at temperatures at which it reacts with the C-S-H, causing carbonarion. However, the total weight of volatiles retained in a C,S paste that has been soaked in methanol and then pumped for about 1 h with a rotary pump is near to that obtained on equilibration with saturated LiCI.H,O (T14). Methanol treatment can thus be used as a rapid, though approximate. method of determining chemically bound water. Other organic liquids could possibly be used in a similar way. In a procedure known as 'D-drying', the sample is equilibrated with ice at - 79•‹Cby continuous evacuation with a rotary pump through a trap cooled in a mixture of solid CO, and ethanol (C20). The partial pressure of water torr. Heating to constant weight at 105 C in an vapour is 5 x atmosphere of uncontrolled humidity, but free from CO,, reduces the water content to approximately the same value, although it is not a strictly defined drying condition. The corresponding temperature on a TG curve obtained in dry and C0,-free N, at IOdegCmin-' is about 145"C, but depends somewhat on the rate of gas flow and design of the apparatus. Water retained in pastes subjected to D-drying or equivalent procedures is known as 'non-evaporable' water. Contrary to some statements in the literature. i t is not a measure of chemically bound water, as much water is lost from interlayer spaces under these conditions. It can nevertheless be used as an empirical measure of the degree of reaction. Typical values in the literature for the water contents of fully reacted C3S pastes dried by these procedures are 20.4-22.0% (OIO), corresponding to C-S-H compositions of 1.7Ca01.3-1.5H,O. Powers and Brownyard (P20) called the water lost from the C-S-H on passing from the saturated to the D-dry condition 'gel water'. I t comprises part of the pore water plus an arbitrarily defined fraction of the interlayer water. Whatever procedure is used to dry or equilibrate a paste, C0,-free conditions are essential, and the amount of water retained can be obtained accurately only if the ignition loss and CO, content are also determined. Calculations based on the initial w/s ratio are often unreliable due to changes in the water content during curing.
5.2.3 X-ray microanalysis and analytical electron microscopy
SEM of polished sections of mature C,S or p-C,S pastes shows well-defined
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regions of C-S-H and CH (Section 5.3.1). Taylor and Nen,bury (Tl8) reviewed X-ray microanalysis (SEM or EPMA) studies of such sections and reported new results. The recorded CalSi ratios for the C-S-H vary irregularly from point to point and increase with the voltage used to accelerate the electrons. As in all analyses of heterogeneous materials by this method, the choice of voltage rests on a compromise: i t must be high enough to excite all the elements adequately, but low enough to produce an interaction volume sufficiently small that single phases can be analysed. The mean or median Ca/Si ratios reported for C,S pastes range from 1.4 at 6 kV to around 2.0 at 25 kV. It is questionable whether 6 kV is sufficient to ensure satisfactory analysis for calcium, and at 25 kV the volume analysed, taking into account the effect of porosity, is several micrometres in each direction, which may be too large to ensure absence of CH or unreacted C,S. Working at 10 kV, Taylor and Newbury obtained mean values of 1.73 and 1.78 for mature pastes of C,S and P-C2S, respectively; these values are near to those obtained using the indirect methods described in the previous section. The range of values was approximately 1.5-2.0 in both cases. The totals obtained in SEM or EPMA microanalyses of C-S-H in C,S or P-C2S pastes, after including oxygen equivalent to the calciun~and silicon, should in theory fall below 100% by an amount equal to the water content under the conditions existing during the analysis, i.e. high vacuum and electron bombardment. The evidence from thermal dehydration and intensive drying at room temperatures (Section 5.2.2) indicates that this content is unlikely to exceed about 15% of the weight of material analysed, implying an analysis total of at least 85%. The totals are sometimes as high as this, but are more often 70-75%, and sometimes even lower (T18). A similar effect is observed with the C-S-H of cement pastes (Section 7.2.5), and appears to increase with the porosity of the region analysed. I t has not been fully explained, and more than one cause may operate. Organic material remaining from liquids used in preparing the polished section. or carbonaceous residues formed from it under the action of the electron beam, may remain in the pores. Superficial carbonation is a possibility (R15). Electrons may lose energy through some ill-understood effect during passage through microporous material (H26). Analytical electron microscopy (X-ray microanalysis in transmission)' would be expected to allow smaller volumes, in the order of tens of nanometres in each direction, to be analysed. The volume analysed is not significantly affected by the exciting voltage within the range (80-200 kV) typically used. Existing techniques give atom ratios but not absolute contents of individual elements. Early studies on ground and redispersed samples of C,S and P-C2S pastes (e.g. G40,M38) indicated variability of Ca/ Si ratio on a micrometre scale, with ranges of 1.2-2.0 for C,S pastes and 1.1-
Hvdration of the Calc~urnSilicate Phases
1 33
1.6 for P-C,S pastes and means of 1.5 and 1.4, respectively. In a study using ion-thinned sections, Groves et 01. (G35) found that a high-resolution probe caused the apparent Ca/Si ratio to decrease with time. The problem was avoided by scanning a 5 - ~ msquare raster, or by using a high-voltage TEM, which gave a spot size of comparable dimensions. The mean values of Ca/Si thus obtained, for three groups of analyses, were 1.66-1.93, and the earlier values of 1 .+I .5 are thus probably incorrect. The marked local variability of Ca/Si ratio was confirmed. No significant compositional differences were observed between 'inner' and 'outer' products (Section 5.3.1). Crystals approximately 100 nm in size, identified by electron diffraction as calcite, were present.
5.3 Structural data for C,S and P-C,S pastes 5.3.1
Microstructure
The first effective studies of microstructural development in calcium silicate or cement pastes were made using light microscopy of thin sections (B5 1). They showed that the CH grows from relatively few centres in the waterfilled space, where i t forms isolated masses typically some tens of micrometres in size. Much more CH is observed with C,S than with P-C,S. At the same time, gel forms around the anhydrous grains and spreads into the remainder of the water-filled space. Early TEM studies, made on ground and redispersed material, showed the presence of acicular and rounded, platey particles, but could not show where these came from in the microstructure (G4 1,C2 1). SEM studies of fracture surfaces (C22,W 18,D13,G42) confirmed these conclusions and showed also that the gel formed in situ from the larger anhydrous grains, called 'inner product', differed in texture from the 'outer product' formed in the water-filled space. It was massive and seemingly almost structureless, whereas the outer product appeared to form columns or fibres radiating from the anhydrous grains. Diamond (D13) distinguished four morphological types of C-S-H gel visible on fracture surfaces of cement pastes; similar forms have been observed in calcium silicate pastes (Fig. 5.4). Type I, prominent at early ages, was the fibrous material, the fibres being up to about 2 pm long. Type 11, described as forming honeycombs or reticular networks, was considered to be very rare in pure calcium silicate pastes, but later work shows that material resembling it is a normal early product. Type 111, prominent in older pastes, was a more massive form, apparently consisting of tightly packed equant grains up to 300 nm across. Type IV, still more featureless and massive, was the inner product, and was also observed in older pastes.
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Fig. 5.4 (A.8) Types I and 11 C-S-H, respectively (SEM of fracture surfaces; courtesy K. L. Scrivener). (C,D) SEMISTEM pair of ion beam thinned section. showing Type 111 C-S-H (top, right) and Type IV C-S-H (top. left and bottom. right; Jennings et a1 (JIO)). (A) is of an ordinary Portland cement paste, w/c = 0.5, aged 10 h. (8) is of a paste of an oil well cement, w/c = 0.44, with 2.4% of CaCI, on the weight of cement, aged I day. (C) and (D) are of a C,S paste. w:c = 0.47, aged 330 dais.
Hydration of the Calcium SI/K,ate Phases
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Cement Chemistry
SEM of fracture surfaces has thus provided valuable information, but its utility is limited by the inherently unrepresentative nature of a fracture surface and by the effects of dehydration in the high vacuum of the instrument. Further information has come from the use of other techniques, especially high-voltage transmission electron microscopy (HVTEM) with environmental or 'wet' cells to reduce water loss (D14,Jl I.G43), examination of ion-thinned sections by TEM or scanning transmission electron microscopy (STEM) (D 15,G35), and BE1 of polished sections with detectors that minimize contrast due to topography and maximize that due to compositional differences (S28,S38,S39). Scrivener and Pratt (S40) briefly described these techniques. Other methods have included X-ray imaging of polished surfaces (T18,T19), SEM of polished surfaces impregnated and etched to reveal porosity (512,513) and the use of a cry0 stage in SEM to avoid dehydration (B.52). Each of these methods has its own limitations, but together they are beginning to yield a reasonably clear picture. Unless a wet cell or cryo stage is used, the fine microstructure is much altered by dehydration in the instrument (J10,S41). However, localized drying occurs in any paste even before it is placed in a high vacuum, as soon as the RH falls below saturation. The water is lost initially from the wider pores, which are probably represented disproportionately on fracture surfaces. The state of the cement paste in a practical concrete may thus vary on both a macro and a micro scale between dry and saturated. Jennings er a/. (JIO) distinguished three principal stages of product formation in C3S pastes: an early product formed during the first 4 h, a middle product formed at 4-24 h, and a late product formed subsequently. Several workers studying the early hydration of C,S in pastes or compacted powders placed in water have described the early product as consisting of foils, flakes or honeycombs, which appear to exfoliate from the C3S surface, The honeyprobably together with some CH (Fl5,M39,M40,JlO,S42,G36). comb material is similar to 'Type 11' C-S-H, and the foils have also been called 'Type E' C-S-H (J10). On drying, they collapse, crumple or roll up, forming fibres (JIO). Broadly similar results have been obtained for C,S hydrated in CH solution (M40) or in the presence of gypsum (M41), sodium silicate or sodium aluminate (514). The early hydration of P-C,S appears to follow a similar course to that of C3S, but reaction is slower and begins preferentially at grain boundaries and exsolution lamellae (M42,G43). Menetrier et a/. (M39,M40) reported that the C3S surfaces were attacked unevenly, and that the first detectable product consisted of minute particles, some of which were probably CH. They found no continuous layer of product. Such layers have been reported, but probably only at w/s ratios above those of normal pastes (G44,L19,G36). Ings et al. (17) placed single crystals of C3S in an alite slurry with w/s = 2, and later removed them for
Hydration of the Calcium Silicate Phases
137
SEM examination. A layer of apparently structureless material built up to a thickness of 5 pm in 30 min, together with cryptocrystalline CH; this later shrank, and acicular outgrowths were formed. The product layer contained aluminium, and had thus formed in part from the surroundirig alite. The middle stage of hydration is characterized by rapid formation of CS-H and CH. In fracture surfaces of calcium silicate pastes, Type I C-S-H is prominent, but HVTEM using wet cells (S39) showed the undried product at 1 day to consist of foils, with no evidence of fibrillar outgrowths. which were secondary forms resulting from ageing or drying. The latter will certainly occur unless a wet cell or cry0 stage is used, and may do so even if i t is, since ageing may cause local exhaustion of water. Fibrillar material may also form during fracture (J 10). The characteristic products of the late stage of hydration are Types 111 and IV C-S-H, and more CH. STEM examination of Type I11 material in ion-thinned section (J10) shows that it, too, consists of interlocked and interleaved thin foils (Fig. 5.4). Type IV material is almost featureless even at the 100-nm level, though a fine pore structure has been observed (D13,G35,G36). Types I, 11, 111 and E C-S-H thus all appear to have an underlying foil morphology, which may be modified or disguised by compaction or drying, and this could well apply also to Type IV. The pastes may therefore contain only one type of C-S-H at the nanometre level. Against this. a less structured form of C-S-H has been observed as the very first product at high w/s ratios. A similar material, described as a viscous, gelatinous product and called 'Type 0' C-S-H, has also been reported to precede the formation of the other varieties in the middle stage of hydration (J10). BE1 of pastes of C,S mixed with C,A shows that no spaces develop between the hydrating C,S particles and the surrounding hydrate (S41). In this respect, C,S differs from Portland cement (Section 7.4.2).
5.3.2
Silicate anion structure
Because C-S-H gel is nearly amorphous, X-ray diffraction has given only very general indications of its structure. The nature of the silicate anions has been determined from the kinetics of the reaction with molybdate (S43), and, in greater detail, by trimethylsilylation (TMS) and "Si NMR. In TMS methods, the sample is treated with a reagent that converts the silicate anions into the corresponding silicic acids, which then react further with replacement of SiOH by SiOSi(CH,),. The resulting TMS derivatives can be identified and semiquantitatively determined by various procedures, of which the most widely used have been differential evaporation to isolate the
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Cement Chemistry
higher molecular weight species, gas liquid chromatography (GLC) and gel permeation chromatography (GPC). It has to be assumed that conversion of the silicic acids into their T M S derivatives is sufficiently rapid to preclude side reactions in which they are altered by hydrolysis or condensation. In practice, these side reactions have never been totally eliminated, though the technique of Tamas et al. (T12) was a major advance on that originally used by Lentz (L20). Because of side reactions, and because it is rarely possible to account for more than 80-90% of the total silicon, the absolute accuracy of percentages of individual anionic species is at best a few per cent. In chemical formulae of silicate ions in the following discussion. negative charges and any attached hydrogen atoms are omitted. Most of the investigations on C,S pastes hydrated for more than a few hours at 15-25'C have shown that any monomer (SiO,) present is attributable within experimental error to unreacted C,S, and that the products formed during the first few days contain dimer (Si,O,), which is first supplemented, and later replaced, by larger species, collectively called polymer (L20,T12, B53, D 16, M43, H27,W19,M44,M45). Fig. 5.5 gives typical results. The percentage of the silicon present as dimer passes through a maximum at about 6 months, but even after 20-30 years is still around 40•‹h.
TIME
(
log scale )
days
years
Fig. 5.5 Percentages of the total silicon in C,S or alite pastes recovered as
monomer (open symbols), dimer (half-open symbols) and polymer (closed symbols) by a trimethylsilylation method. Differently shaped symbols denote different C,S or alite specimens. Mohan and Taylor (M43).
Hydration of the Calcium Silicate Phases
139
GLC, GPC, mass spectrometry and chemical analyses of TMS derivatives established the nature and size distribution of anion sizes within the polymer (D16,U9,M43,H27). From chemical analysis, one may calculate the mean connectivity of the SiO, tetrahedra, i.e. the number of other SiO, tetrahedra with which each one shares oxygen atoms; typical values are 1.6 2 . 0 (M43,M44). The anions are linear, with pentamer (Si,O,,) the most important single component at all ages, the next most important being probably linear octamer (Si,O,,). As the paste ages, the mean size of the anions increases, this process continuing long after all the C,S has reacted. At ages up to 6 months, the polymer fraction consists largely of pentamer and octamer, and even at 20-30 years these species account for a substantial fraction of the total silicon, though larger anions, containing some tens of SiO, tetrahedra, are also present (M43). Studies at 2-5•‹C have given divergent results. Two investigations showed more monomer at early ages than could be attributed to unreacted C,S. suggesting that the initial product formed at higher temperatures also contained monomeric silicate ions, which rapidly dimerized (B53,S43), but later investigations did not confirm this (P21,H27). With increase in temperature above ambient, condensation is increasingly rapid, and by 65' C larger anions, containing 2 100 SiO, tetrahedra, are formed (B53,S43,H27). Early conclusions that admixtures, such as CaCI,, affect thedegree ofpolymerization for a given degree of reaction of the C,S were not confirmed (H27). The behaviour of P-C,S is similar to that of C,S, apart from the rate of reaction (M44). A conclusion that three-dimensional cluster ions are formed (C23) is incompatible with the observed connectivities and with the NMR evidence discussed in the next paragraph, and can probably be attributed to occurrence of side reactions. The use of 29Si 'magic angle' NMR complements the trimethylsilylation method in two ways. First, it is a purely physical technique, in which there is no possibility of altering the structure through side reactions. Except with very young samples, no preliminary drying is needed. Second, it gives slightly different data, relating not directly to the fractions of the silicon present in different anionic species, but to the fractions present in different environments. The terms Q O . .. Q4 refer to the connectivities, Q0 thus denoting isolated tetrahedra, Q1 end group tetrahedra (and thus including silicon present as Si,O,), Q2 middle groups, and so on. For C,S pastes, the content of Q0 begins to decrease after a few hours, with formation of Q'. later accompanied by Q2; no Q3 or Q4 is detected (L21,C24,R16). This result is compatible with the formation of dimer, later accompanied or replaced by either single chains or rings, but not with that of double chains, clusters or other more complex species. Ciayden et al. (C24) found high correlations between the relative intensities of the Q0 and Q' peaks in the
140
Cement Chemistry
NMR spectrum, the amount of C H given by T G and the cumulative heat evolution. The TMS method is insufficiently precise to exclude the possibility that the hydration products include a small fraction of monomer. Cross-polarization NMR can distinguish between silicon atoms in tetrahedra that carry one or more hydrogen atoms and ones that d o not, and thus between lnonorner present in the anhydrous compounds and that present in a hydration product, which is almost certain to carry hydrogen. Using this technique, Young (Y6) found only small amounts of hydrated monomer in C3S pastes cured a t 2-65"C, which he attributed to water adsorbed on the C3S surface before mixing. In contrast to this observation, Rodger ct 01. (R16) found the product formed during the first few hours in a C,S paste to contain only monomeric anions, dimer only beginning to form later. The monomer did not disappear when the larger species were formed; it accounted for 2% of the silicon at 24 h and was still detectable at 6 months. The initial formation of monomer agrees qualitatively with some of the results at low temperatures mentioned earlier (B53S43). Early conclusions from T M S studies that the average size of the silicate anions in pastes increases on drying (M46,B54) were not supported by later work (P21). An N M R study on the effect of drying conditions on the anion structures of C-S-H prepared in suspension (M47) (Section 5.4.4) has important implications for pastes.
5.3.3 X-ray diffraction pattern, densities and other data Fig. 5.6 shows the X R D powder pattern of a 23-year-old paste of p-C,S. Patterns of fully reacted C,S pastes are similar, except that the CH peaks are relatively more intense. The only effects definitely attributable to C-S-H are the diffuse peak a t 0.27-0.31 nm and the somewhat sharper one at 0.182 n Attempts to obtain selected area electron diffraction patterns from the C-SH of calcium silicate or cement pastes have usually failed, but, occasionally, particles present in ground and redispersed samples have yielded poorly defined patterns (G41,C25) (Section 5.4.6). A later study by this metho (M48) has been severely, and in the writer's opinion justifiably, criticize ((345). The density of C-S-H gel depends on the water content. For strong1 dried samples, i t also depends on how the solid volume is defined, causin observed values to vary with the fluid used in their determination. Widely differing values can thus be equally meaningful, but for a given application the appropriate one must be used. Since the C-S-H in C,S or 0-C,S pastes is mixed with C H , it is necessary either to correct for this phase (D =
Hydration of the Calcium Silicate Phases
141
2242 kg IT-^) or to remove it. A procedure for the latter has been described ( R 14). At about 1 I % RH on first drying, the interlayer spaces are full of water and larger spaces are empty; H,O/SiO, is approximately 2.0 (Section 5.2.2). Using helium pyknometry, Feldman (F16) obtained values of 23502360 kg n1-3 for fully reacted C3S pastes; correction for CH gave 24302450 kg m - 3 for the C-S-H. Determinations on C-S-H samples made by hydrating C3S in suspension showed that substitution of methanol or aqueous CH for helium as the fluid had no significant effect on the result. For a fully hydrated C3S paste with w/s = 0.4 from which the CH had been removed, Hansen (H28) found 2180 kg m-3. For all these values, the empty pores are excluded from the solid volume.
Fig. 5.6 X R D powder diffraction pattern (microdensitometcrtrace of Guinier film: CuK,,, radiation: spacings in A) of a fully reacted P-C,S paste. Mohan and Taylor (M38).
The C-S-H present in a saturated paste is assumed to include that part of the porosity, and its content of water, without which it cannot be formed. The H,O/SiO, ratio is approximately 4.0. Assuming the water in excess of that present at 1 1 % RH to have a density of 1000 kgm-3, Hansen (H28) calculated the density of the C-S-H to be 1850-1900 kgm-3. This value is near those of 1900-2100 kg m - 3 for cement pastes under saturated conditions (P20), if allowance is made for the CH and other hydrated phases which these contain. Helium pyknometry of D-dried samples has given values of 23202350 kg m - for C3S pastes (F 16) and of 2440 kg m - for cement pastes (P20); correction for CH and other phases suggests values of 24002500 kg m - 3 for the C-S-H. For a C3S paste with w/s 0.4 from which the CH had been removed, Hansen (H28) found 2390 kg m-3. With D-dried samples, methanol gives similar results to helium, but water or CH solution give much higher values, Brunauer and Greenberg (B55) obtaining values around 2860 kg m-3, after correction for CH, for the C-S-H of C3S and PC,S pastes. Determinations using water on D-dried cement pastes, based on the difference between the weights of the D-dried and saturated materials, typically give values, uncorrected for CH, of 2500-2600 kg m-3. Relis and
142
Cement Chemistrv
Soroka (RI 7) found these to increase with the degree of hydration. an upper limit of 2700 kg m 3 being found for a specific cement. Feldman (FIG) explained the difference between the values for D-dried samples obtained with aqueous and other fluids. When the material is dried, the layers partially collapse, and some empty spaces remain between them. Water penetrates into this space, and the density obtained, after correction for C H and any other phases present, is that of the layers themselves. Organic liquids d o not penetrate, and helium penetrates only slowly. The empty interlayer space is counted as part of the solid volume. with a consequent decrease in the measured density. The IR absorption spectra of C,S, alitc or P-C,S, pastes (H29.L22,S44) are relatively diffuse, as would be expected from the poor crystallinity. Si-0 stretching is represented by a single, broad band peaking at about 970 cm-I; this is compatible with the existence of condensed silicate anions. Small amounts of carbonate are readily detectable by the absorpt~on near 1400 c m - '. 5.4 Structural models for C-S-H g e l 5.4.1
General
Many crystalline calcium silicate hydrates are known. Most are formed only under hydrothermal conditions, i.e. in aqueous systems under pressure above 100•‹C(Section 11.7), but afwillite (C3S,H3) is probably the stable phase in contact with C H and solution at ordinary temperatures (L23). It can be prepared from C3S by ball milling with water at ordinary temperature (K14) or seeding (D17), but shows no similarities to C-S--H gel and nothing resembling it is formed in normal cement pastes. Two other crystalline phases that can be prepared in aqueous suspensions below IOO'C show much closer relationships. These are 1.4-nm tobermorite (C,S,H, approx.) and jennite (C,S,H, ,). Various types of semicrystalline calcium sil~cate hydrate are intermediate in structure between these compounds and C-S-H gel. Two relatively well defined ones are C-S-H(1) and C--5-H(I1). which are closely related to 1.4-nm tobermorite and jennite, respectively. A gelatinous calcium silicate hydrate, called plombierite, occurs in nature: i t is of variable composition, and some specimens resemble C-S-H(1) (M49). 5.4.2
1.4-nm tobermorite and jennite
1.4-nm tobermorite has a layer structure and the prefix relates to the layer thickness. On being heated a t 55OC, it loses interlayer water and undergoes
Hydration of the Calcium Silicate Phases
143
unidimensional lattice shrinkage, giving I . l -nni tobermorite (C,S,H, approx.), often called, simply, tobermorite. 1.4-nm tobermorite occurs as a natural mineral (F17) and may be synthesized from C H and silicic acid in aqueous suspensions at 60•‹C(K 15,H30). Table 5.2 gives crystal data. The crystal structure has not been directly determined, but may be inferred from that of I . I-nm tobermorite (M50,H3 I ) , aided by evidence from dehydration and IR studies (F 17), broad line proton NMR and silicate anion dctermination by the molybdate method (W20) and "Si N M R (W21.KI6). Each layer consists of a central part of empirical formula CaO,, of which all the oxygen atoms are shared with Si-O chains, which form ribs covering each surface; between these layers are water molecules and additional calcium atoms (Fig. 5.7A and B). The idealized constitutional formula is possibly Ca5(Si,0,,H2).8H,0, but the atomic contents of the unit cell of the natural mineral show that only 5.5 silicon atoms are present where 6 would be expected (M51), and synthetic studies also indicate that the Ca/Si ratio is higher than 0.8 (H30). Many silicate tetrahedra thus appear to be missing from the chains, and the actual formula may be nearer to Ca,Si,.,0,,H2.8H20, corresponding to a mean chain length of I I tetrahedra. The conclusion from 29Si N M R (K16) that little SiOH is present appears to be of doubtful validity. Table 5.2
Crystal data for 1.4-nm tobermorite, jennite and related phases
Phase Molar ratios CaO SiO, H2O Pseudocell parameters (nm) b (nm) c (nm) a
fi Y Lattice type Z
Dx (kgm-3)
Reference
1.4-nm tobermorite C-S-H( I ) Jennite
C-S-H( I I )
5 5.5
9 0.5264 0.3670 2.797 90.0" 90.0" 90.0"
0.560 0.364 2.5 90.0" 90.0" 90.0"
1 I
I I
2224 F17, T5
2250 T20, T5
0.996 0.364 2.136 91 .8" 101.8" 89.6" A I
2332 G46, T5
0.993 0.364 2.036 90.0 106.1' 90.0 A I
2350 G47. T5
All data are for the pseudocells and compositions stated. The true cells have doubled values of h and can be in varying degrees disordered: a and c must often be differently defined. The structure of C-S-H(I) has little more than two-dimensional order, and that of C S H(I1) has less than full three-dimensional order.
Hydration of the Calcium Silicate Phases
145
The Ca-0 parts of the layers have a pse~idohexagonal,centred lattice with a = 0.56 nm, h = 0.36 nm, and may be regarded as layers of C H extremely
distorted due to replacement of their O H groups by SiO, tetrahedra. the 0-0 distances of which have to be satisfied. CH, referred to orthogonal axes, has rr = 0.62 (0.36 x &), h = 0.36 nm. The Si-0 chains are o f a type called dreierketten, i.e. they are kinked so as to repeat at intervals of three tetrahedra. This conformation is imposed by the coordination requirements of the calcium ion, and occurs in many calciuni silicates, including wollastonite (Section 2.2.1). In 1.4-nm tobermorite, they are single. i.e. not condensed with other similar chains to form double chains or more complex Si0 structures, and have the empirical formula Si,O,H. I . I-nni tobermorite, when formed by dehydration of the 1.4-nni form, ditTers from 1.4-nm tobermorite only in having lost some of its interlayer water n~olecules.and thus also contains single dreierketten. Another variety of it. formed under hydrothermal conditions, contains double chains (Section 11.7.4). Jennite (C,S,H, ,) occurs as a natural mineral (C26,G46). and, like 1.4nm tobermorite, may be synthesized from C H and hydrous silica in aqueous suspensions below 100•‹C (H32,H33). It forms most readily at 80 C from mixes of CH with reactive silica at Ca/Si ratios of 1 .I-- 1.2. though on prolonged reaction these yield 1.4-nin tobermorite (H33). In nature. it occurs in contact with 1.4-nm tobermorite ((26). with which it is probably in metastable equilibrium. Like 1.4-nni tobermorite, it has a layer structure, and at 70-90•‹C loses water with unidimensional lattice shrinkage. giving metajennite (C,S,H,) (C26,H32). The layer thickness is 1.05 nm in jennite and 0.87 nm in metajennite ((346). Table 5.2 includes crystal data for both phases. The crystal structure has not been determined, but its major features (Fig. 5.7C) may be inferred from the crystal data, dehydration behaviour, I R spectral, 29Si NMR and other evidence. As with 1.4-nm tobermorite. N M R evidence against the presence of SiOH groups is weak, and the constitutional formula is probably Ca,(Si,0,,H,)(OH),~6H20, the structure somewhat resembling that of 1.4-nm tobermorite but with alternate chains replaced by rows of O H groups. The unit cell parameters show that while each layer could be based on a central part of empirical formula CaO,, the distortion of the C H layer needed to produce it would differ greatly from that in 1.4-nm tobermorite. It is probably folded into corrugations parallel to the h axis (T21).
5.4.3 C-S-H(I)
and similar materials
Attempts to synthesize 1.4-nm tobermorite or jennite from aqueous suspensions at ordinary temperatures normally yield semicrystalline products.
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These vary in crystallinity from ones whose XRD powder patterns approach those of 1.4-nm tobermorite or, much less frequently, jennite. to materials no more ordered than those formed in pastes. Reaction between CH and hydrous silica gives a product called C-S-H(l), first adequately described by zur Strassen and Stratling (Z9) and more fully by Taylor (T20). Similar but often less crystalline products are formed on mixing solutions of sodium silicate and calcium salts, or under certain conditions from C,S (T20,B56,J 15,G48) and also as initial or intermediate products of hydrothermal reactions at temperatures up to at least 180•‹C.Table 5.3 gives the XRD powder pattern of a relatively highly crystalline specimen of C-S-H(1). It consists largely of the hkO reflections, or hk band heads, of 1.4-nm tobermorite, together with a broad basal reflection. The latter. which is absent in less ordered forms, corresponds to the mean layer thickness. C-S--H(I) may be prepared with Ca/Si ratios varying from 0.8 to about 1.5. With increase in Ca/Si ratio, the degree of crystallinity tends to decrease. and the basal spacing, if present, also decreases, from about 1.3 nm at Ca/Si = 0.8 to about I . I nm at Ca/Si = 1.3 (T22). The basal spacing also decreases on heating. values of 0.91-1.13 nm being obtained at 108•‹C(T23). TEM shows C-S-H(1) to consist of crumpled foils a few nanometres thick. with a tendency to elongation or fibrous character at the higher CasSi ratios ((341). Table 5.3 X-ray powder diffract~ondata for C-S-H(1) tl(nm)I,,,
hkl
d(nm)I,,,
vs
002
0.24 0.21 0.182 0.167
1.25 0.53 0.304 0.280
vvw 10. vs 11. s
20.
hkl
w,d ? w,d ? 02. s mw 31.
d(nm)I,,,
hkl
d(nm)I,,,
hkl
0.152 vw 0.140~ 0.123 vw 0.117 vw
22. 40. ? 13.
0.1 1 1 w 42. 0 . 1 0 7 ~ 51. ~
I'attcrn is for a relatively highly crystalline variety; all peaks are broad. s = strong. \v = weak, moderately, v = very, d = diffuse. Indices relate to pseudocell in Table 5.2; hkl denolesa hand head given by an essentially two-dimensional lattice (T22).
111 =
Stade and co-workers (S45SS47) studied silicate anion structures in preparations of 'C-S-H(di,poly)', obtained by reactions in suspension at either 80•‹C or 150•‹Cand which gave XRD patterns generally similar to those of C-S-H(1). They used the molybdate and TMS methods and 29Si NMR. The results showed that for Ca/Si ratios of 1.1-1.5. the anion size distribution varied little with either Ca/Si ratio or temperature of preparation, 40-50% of the silicon being in dimeric, and 50-60% in polymeric, single chain anions. At Ca/Si = 1.0, the average anion size was greater, and at Ca/Si = 2.0, a higher proportion of end groups, found using NMR, indicated an increased proportion of dimer. Hydroxyl groups were attached
Hydration of the Calcium Silicate Phases
147
to both end group (Q1) and middle group (Q2) tetrahedra. Changes in anion structure on heating were also studied. The silicate anion structures of C-S-H(1) preparations appear to be affected by how long the material remains in contact with its mother liquor and by how strongly it is subsequently dried. Experiments using the [nolybdate method showed that the anions in precipitates made from CaCI, and sodium silicate solutions at 0•‹Cwere mainly those present in the silicate solution, and thus monomeric if the latter is sufficiently dilute (S45). By allowing such products to stand in contact with their mother liquors at O'C. and drying at - 1O0C, preparations with Ca/Si ratios of 1.2-1.5 were obtained that contained only dimeric silicate anions. Macphee 01. (M47) made an NMR study of C-S-H samples prepared in aqueous suspension. In preparations with Ca/Si = 1.8 that had been in contact with mother liquor for 88 weeks the anions were almost wholly dimeric, irrespective of drying condition between 10% and 50% R H , but in one with Ca/Si = 1.1 that had been in contact with the solution for only 4 weeks the ratio of Q2 to Q' increased on drying. The C-S-H with Ca/Si = 1.8 thus differed markedly in silicate anion structure from that of similar composition formed in C,S pastes. The authors suggested that, in freshly formed C-S-H. drying caused increased condensation. If the material was formed in suspension, the free supply of water ensured that little condensation beyond dimer occurred in the presence of the mother liquor, and the aged material was less susceptible to the effect of drying. In a C,S or cement paste, the demand for water caused localized drying in the inner product. with a consequent increase in condensation. The inner or late product could thus be less hydrated and more highly condensed than the outer product. DTA curves of C-S-H(1) preparations show endotherms at 100-200'C and exotherms at 835-900•‹C, the latter temperatures increasing with Ca/Si ratio (K 17). C-S-H(di,poly) preparations gave somewhat similar results (S46), but the temperature of the exotherm varied erratically with CaiSi ratio and, except at Ca/Si = 0.8, an additional endotherm occurred at 332372•‹C. Solvents that dissolve lime, such as ethyl acetoacetate, have little action on preparations with Ca/Si ratios of 1.25-1.33, but remove CaO from ones of higher Ca/Si ratio until that composition is reached (K17,S46).
5.4.4
Products formed in suspensions from C,S or p-C,S
Many studies have been reported on the hydration of C,S or P-C2S in aqueous suspensions, often termed 'bottle hydration'. If the reaction is prolonged, so that no unreacted starting material remains, and the w/s ratio is sufficiently high that the Ca/Si ratio of the product is below about 1.5, the
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Cement Chemistry
product is C-S--H(I), essentially similar to that obtained by other methods (T20). At lower w/s ratios, such that the product contains CH. the C-S--H is similar in composition and crystallinity to that formed in pastes (F13). The products of bottle hydration for short times, such that unreacted starting material remains. are discussed in Section 5.5.1. Under some conditions, prolonged reaction at room temperature gives a product differing from any of the above, which has been called C-S-H(I1). This was obtained from C,S by shaking with water, and repeatedly removing proportions of the liquid until the bulk Ca/Si ratio had fallen to about 2.0 (T20), and by bottle hydration of p-C,S (B55). I t has a Ca Si ratio somewhat below 2.0, is seen by TEM to consist of bundles of fibres ((341) and gives a characteristic X R D powder pattern (Table 5.4). which differs from that of C-S-H(1) and shows similarities to that ofjennitc. The unit cell parameters, determined by electron diffraction, are also unlike those of 1.4nm tobermorite or C-S-H(I) and near those of jennite (Table 5.2). The silicate anion structure comprises dimeric and single chain anions in cornparable proportions, and the structure as a whole appears to be derived from that of jennite by omission of many tetrahedra from the silicate chains, giving a constitutional formula approximating to Ca,(Si,0,H)(Si20,H)(OH),.6H2O (G47). Attempts to repeat these preparations have failed, and the conditions giving C-S-H(I1) are obscure. Table 5.4 X-ray powdcr diffraction data for C-S--H(II) t/(nm) I,,,
hkl
c/(nm) I,,,
hkl
d(nm)
I,,,
hkl
All peaks are broad; indexing, which relates to the pseudocell in Table 5.2. is approximate as order is not fully three-diniensional. Data arc from Ref. PI. Card 29-374, with peaks of I,,, < 5 omitted.
5.4.5 Structural relationships The X R D powder pattern of C-S-H gel (Fig. 5.6) suggests structural similar~tiesto 1.4-nm tobermorite and jennite. The band at 0.27-0.31 nm and the peak at 0.182nm could be derived from the sharper peaks in ' corresponding parts of the patterns of either or both of those phases. The j
Hydration of the Calcium Silicate Phases
149
patterns of C-S-H(1) and C-S-H(II), which are phases of intermediate crystallinity, provide links and thus strengthen the hypothesis. The C-S-H(1) peaks at 0.304nm, 0.280nm and 0.182nm (Table 5.3) represent the three shortest repeat distances in the plane of a distorted CH layer (compare Fig. 5.1). and those in the same regions of the C-S-H(I1) pattern have a similar significance for a different type of distortion. Slegers c.1 (11. (S44) attempted to interpret the radial distribution function; this approach might repay further investigation. Early studies suggested that the increase in Ca/Si ratio in C-S-H(1) above that of tobermorite could arise from a combination of omission of tetrahedra from the chains and incorporation of additional calcium in the interlayer (T23). In a dreierkette (Fig. 5.8), the tetrahedra are of two types. which we shall call 'paired' and 'bridging'. In calcium silicate structures, the paired tetrahedra share oxygen atoms with columns of Ca-0 polyhedra, which in 1.4-nm tobermorite and, by analogy, also in jennite, are themselves condensed into CaO, layers. If all the bridging tetrahedra are missing, a series of dimeric anions results; more generally, if some or all are missing, chains containing 2, 5, 8, (3n - 1) silicon atoms result (G47.Tl5). This accounts for the observed sequence of chain lengths in C,S pastes. The hypothesis that increase in Ca/Si ratio is due to removal of parts of the structure is also consistent with the observation that it is associated with decrease in crystallinity and layer thickness.
Fig. 5.8 Silicate chain of the type present in 1.4-nm tobermorite and jennite (dreierkette). In the tobermorite structure, the oxygen atoms at the bottom of the figure are also part of the central CaO, layer. The tetrahedra in the lower row are described as paired and those in the upper row as bridging. A bridging tetrahedron is missing (at X). Suggested positions of hydrogen atoms and negative charges balanced by interlayer cations are included. Taylor (T24).
At one time, the relation between C-S-H gel and tobermorite was widely believed to be very close, and the name 'tobermorite gel' was in common use. The Ca/Si ratio and poor crystallinity of C-S-H gel do not justify this name. Assuming the idealized formula C,S,H, for 1.4-nm tobermorite, removal of
1 50
Cement Chemistry
all the bridging tetrahedra would only raise the Ca/Si ratio to 1.25. and in pastes more than a few days old, not all of these tetrahedra arc absent. Omission of tetrahedra is therefore not the only cause of increased Ca/Si ratio. The other cause can only be incorporation of extra Cn" ions, which could be balanced by omission of H + or incorporation of O H - or both. From their studies on 'C-S-H(di,poly)', Stade and Wieker (S45) concluded that C a 2 + and OH- ions could both be present i n the interlayer region of a tobermorite-like structure. Stade (S46) suggested that in the products containing both dimeric and polymeric ions, one surface of each tobermorite-type layer was composed of dimeric, and the other of polymeric, ions, thus accounting for the observed near-constancy of dimer/ polymer ratio. In the purely dimeric material isolated at - 10•‹C. both surfaces were composed of dimeric ions. From the crystal chemical standpoint, it is difficult to see how anions. as well as cations, could be present in the interlayer of a tobermorite-type structure. Another possibility is that regions of jennite-type structure are present (T21,T24). If the structure suggested in Section 5.4.2 for jcnnite is correct, this is equivalent from the compositional standpoint to adding the elements of CH to a tobermorite-type structure. The difference between this hypothesis and that of Stade and Wieker may be largely semantic. since the probable corrugation of the layers in a jennite-type structure could bring the additional Ca2+ and O H - ions into what could alternatively be described as part of the interlayer region.
5.4.6 A mixed tobermorite-jennite-type model for C-S-H gel
Taylor (T24) suggested that C-S-H gel, assumed to be equilibrated at 11% RH, contains elements of structure derived from both 1.4-nm tobermorite and jennite. He suggested that these were present in separate layers, but transition between the structures might also occur within a layer if the boundary is parallel to the chain direction. In Fig. 5.9, the Ca/Si ratio is plotted against a function of chain length for each structure, assuming that omission of bridging tetrahedra is compositionally equivalent to rcmoval of SiO,. The observed Ca/Si ratio of about 1.8 could arise from a mixture of the two structures, both with dimeric anions, or from a jennite-type structure with, on the average, pentameric anions. Transition from the first to the second would explain the observed changes in anion distrib~~tion with time. Because it is a change from a more to a less imperfect structure, it might also explain the direction taken by the process. However, as noted in Section 5.4.3, the increase in mean anion size may be at least in part a consequence of local dehydration.
Hydration of the Calcium Silicate Phases
1.4 nm T o b e r m o r i t e
151
-I
Chain Length
Fig. 5.9 Calculated Ca/Si ratio plotted against a function of chain lcngth for jennitc and 1.4-nm tobermorite modified by omission of bridging tetrahedra. Taylor (T74).
Several other lines of evidence were cited in support of this hypothesis. The T G curve of C-S-H gel (Fig. 5.3), expressed in terms of H,O/Ca ratio, was shown to be intermediate between those of 1.4-nm tobermorite and jennite. The densities and H,O/Ca ratios of C-S-H gel are similar to those of 1.4-nm tobermorite, jennite and structurally related minerals of comparable H,O/Ca ratios (Table 5.5). The XRD evidence has already been noted; of the few selected area electron diffraction patterns that have been obtained from particles of C-S-H gel, some were shown to resemble ones of tobermorite minerals, and others that of C-S-H(I1). Finally, the occurrence of two types of structure, with differing compositions, could explain the local variability in composition observed in electron optical analyses. The same hypothesis can be applied to preparations made in suspension. In C-S-H(I), increase in Ca/Si ratio above the theoretical minimum value of 5 : 6 probably results entirely from omission of bridging tetrahedra until a value of about 1.0 is reached. Relatively highly ordered materials can be obtained. Further increase in Ca/Si ratio could result either from omission of more bridging tetrahedra, o r from the presence of regions of jennite-type
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Cement Chemistry
structure, a given ratio thus being obtainable by different combinations of the two effects, leading to possible variability in properties. The purely dimeric materials would, in general, contain regions of both structural types. C-S-H(1) is thus material predominantly, but not always entirely, of tobermorite-type structure. The mixing of structural types markedly impairs the crystallinity, and at Ca/Si ratios above about 1.5, it is impossible to obtain material giving the tobermorite-type, C-S-H(1) X R D pattern. At higher ratios, preparations sufficiently ordered to give a distinctive pattern give one of C-S-H(II), or, at higher temperatures, jennite. Table 5.5
Water/calcium ratios and observed densities (kg m - ')
Material
H ,OICa
Density
Ref."
C-S-H gel (saturated) Plombierite (0.8Ca03i0,.sH20; saturated) C -S-H gel ( I 1 % RH) Plombierite ( I I % RH) Jcnnitc I .4-nm tobermorite Tacharanite (Ca, ,AI,Si, ,O,,H,,) C-S-H gel ( I 10•‹C) Plombierite ( I 10•‹C) Mctajennite I. l -nm tobermorite Oyelite (Ca,,Si,,B,O,,H,,) "Values are ~n some cases calculated from data in references givcn 'Typical values obtained using water, corrected for CH.
Some other models for the structure of C-S-H gel that have been proposed are incompatible with the evidence. A proposal identifying it with natural tobermorite, based on IR and extraction results (S37), appears to ignore both composition and degree of crystallinity. Another, assuming it to be closely related to the C H structure, with incorporation of monomeric silicate ions (G38), is inconsistent with the observed silicate anion structure. As noted earlier, one assuming three-dimensional anionic clusters (C23) is inconsistent with the 29Si NMR evidence, and with the overwhelming proportion of the other evidence on silicate anion structure. The monomeric material formed as the initial product, either in pastes or in suspensions, is unlikely to have a structure specifically related to that of 1.4-nm tobermorite o r jennite, as both require the presence of dreierketten or Si,O, groups, though its structure might still be based on Ca-0 sheets.
Hydration of the Calcium Silicate Phases
153
5.5 Equilibria 5.5.1 Solubility relations Equilibria in the CaO-Si0,-H,O system at ordinary temperatures have been widely studied for the light they may cast on cement hydration and on leaching of concrete by ground waters. Steinour (S48,S49) reviewed early work and Jennings (J15) discussed later studies. If C-S-H(1) is placed in water or CH solutions, its Ca/Si ratio changes until equilibrium is reached. Except at low CaO concentrations, the SiO, concentrations in solution are very low, so that transfer between solid and solution is almost entirely of CaO. The data may be plotted schematically on a triangular diagram (Fig. 5.10), but because of the low concentrations it is more useful to employ separate plots of the Ca/Si ratio of the solid or Csio2against C,,, (Figs 5.1 1 and 5.12).
NOT TO
/ /
1
solution
Fig. 5.10 The system CaO-Si0,-H,O lor et al. (T2.5).
at ordinary temperatures (schematic).Tay-
154
Cement Chemistry
1.6-0
- 1.2-
.-
0
V)
-
.C 0.8.-
m U
0.4 -
-
-
4 8 12 16 20 24 CaO i n s o l u t i o n ( m m o l e 1-1 ) Fig. 5.11 Metastable equilibrium curve relating the Ca Si ratios of C-S H preparations to the analytical concentration of CaO in the solution. selectcd as typical from the results of many investigations. After Steinour (S49). In most of the investigations, the solid starting materials were either CH and hydrous silica, or C-S-H prepared either from those materials or by mixing solutions of sodium silicate and a soluble calcium salt, followed by washing to remove the soluble product. In some, anhydrous calcium silicates were used. The various sets of data represented by Figs 5.1 I and 5.12 show considerable divergencies; a few studies suggested the existence of a step in the curve of Fig. 5.1 I at Ca/Si I . 1 (S48). With the important exceptions of some of the data obtained using C,S or C,S, none of these differences is clearly related to ones in starting material. They may, nevertheless, be associated with differences in structure, discussed in Section 5.4.6. 'XRD powder evidence shows that the central part of the curve in Fig. 5.11 (C,.,, 2 2-22 mol I - ' ) and curve A of Fig. 5.12 represent the solubilities of C-S-H(1) preparations and that there are invariant points for hydrous silica, I mmol I - ' and C-S-H(1) with Ca/Si 0.8 and solution with C,, C',,,, 2 2 mmol I ! , and for CH, C-S-H(1) with Ca/Si 1.4 and solution with C,;,, -. 22 mmol I - ' and CSio2 1 pm01 I - ' (S49,T20,515). Fig. 5.12 , includes metastable solubility curves for hydrous silica and CH. I1 Jennings (515) noted that observed points on plots of CSioZversus tended to fall on one or other of two curves, with relatively few in between ? (Fig. 5.12). Of those that fell between, some were obtained using P-C2S and others were suspect from carbonation or for other reasons. Those falling on the upper curve, B, had all been obtained by reaction of C,S with water for periods not exceeding a few days; probably in all cases, unreacted C,S was present. Some of the experiments using C,S, and all of those using starting
--
--
-
--
c,,I
Hydration of the Calcium Silicate Phases
155
materials other than C3S or C,S, gave points lying on the lower curve, A. Of the experiments using C,S that gave points on curve A, some were of durations of 4 h or less (B56). but most appear to have been of long duration, and in at least one case, points lying on curve A were obtained only if no unreacted C3S remained (T20). I
I
I
I
1
H Y D R O U S SILICA
0
0
1 1 10 15 20 25 30 CaO CONCENTRATION ( m m o l l-l)
5
.
Fig. 5.12 Concentration data for C-S-H based on the results of many investigations; for explanations of curves, see text. Broken lines give an indication of the scatter of the data. After Jennings (J15).
Curve A is clearly the solubility curve of C-S-H(1); scatter at the CaOrich end may arise because a given Ca/Si ratio can be associated with more than one structural arrangement. However, any form of C-S-H that is structurally derived from 1.4-nm tobermorite or jennite, or perhaps more widely that is based on Ca-0 sheets, probably has a solubility lying on or close to this curve. There are no solubility data for C-S-H(II), 1.4-nm tobermorite or jennite, but one might expect the curves for the crystalline
156
I
Cement Chemistry
phases to lie below the relevant parts of curve A, and that of C-S-H(I1) to lie near or slightly below the high-CaO portion of that same curve. Curve B is most unlikely to represent the metastable solubility of anhyd. rous C,S; thermodynamic calculations (S50) indicate that such a curve would be very much higher, as would be expected from the high reactivity. Jennings (J 15,516) and Gartner and Jennings ((348) considered it to be the metastable solubility curve of a product formed as a layer on the C,S surface. Thermodynamic calculations based on this assumption indicated Ca/Si ratios of 1.1 at low and 1.65 at high CaO concentrations (G48); these values are broadly compatible with ESCA evidence (Section 5.6.2). Barret and Bertrandie (B57,B58) considered that curve B was not a solubility curve, but represented a quasisteady state in which the concentrations were such as to equalize the rate of dissolution of a hydroxylatcd surface layer on the C3S and the rate of precipitation of Both views associate the curve, though in different ways, with the presence of an altered surface layer on the C,S that is metastable relative to the C--S-H whose solubility is represented by curve A. Experiments using P-C,S gave compositions lying on a curve between curves A and B (B57,G49) and which may similarly be associated with an altered surface layer on the P-C,S. There are few data on the concentrations of CaO and SiO, in the pore solutions of C,S pastes (w/s < 1.0). as opposed to suspensions, and those that exist are conflicting as regards C,,,, (B56,Ol 1). The values of C,,, after long times (e.g. 27 mmol 1 - ' ) ( 0 1 1) appear to be significantly above the solubility of CH; this has been attributed to incorporation of SiO, in the CH (W22), but might also be due to the small crystal size of some of this phase.
5.5.2 Species in solution Barby et ul. (B59) summarized data for silicate species in alkali silicate solutions. Monosilicic acid, H4Si04, has pK,,, = 9.46. pK,, = 12.56, and pK,, probably about 15. Monomeric species are overwhelmingly predominant at CSi,, below about 2 mmol 1 at pH values up to 9. and up to higher concentrations at higher pH. The solubility product of CH. in terms of activities, is 9.1 x mo13 at 25•‹C(G39), and the stability constant of the complex CaOHt is 20 mol I - ' (S51). Undissociated CH is not a significant species in saturated CH solution ((348). In saturated CH solution at 25•‹Cthe activities of Ca2' and CaOHt are roughly equal. and the pH is 12.45. The stability constants of CaH,SiO: and CaH,SiO: are reported to be 1240 mol I - ' and 2.5 mol I-', respectively (%I), but some doubt has been cast on these values by the results of thermodynamic calculations using them (G48).
-'
Hydration of the Calcium Silicate Phases
157
Using the equilibrium constants given above, together with standard to calculate ionic strength and activity coefficients, one may calculate the species concentrations and species activities in a system of given bulk composition. For C,,, = 28.0 mmol I-', Csio, = 20.0 pmol 1 - ', and assuming that calcium silicate complexes are not present, the species concentrations (in mol I-') are: C a 2 + , 1.90 x lo-,; CaOH', 8.98 x lo-,; O H - , 4.70 x lo-'; H,Si04, 4.19 x lo-"; H3Si0,, 6.82 x lo-"; H,SiO:-, 1.30 x lo-'; HSiOi-, 1.46 x lo-' (G48). The activity of O H - is 3.78 x 10-2 moll-'; this corresponds to a pH of 12.58. If the complexes c~H,s~O: and CaH3SiOf are assumed to occur, with stability constants of 2000 and 20 mol I-', respectively, the concentrations of the Si0,-containing species become (mol I-'): CaH,SiO:, 2.01 x lo-'; CaH,SiO:, 1.61 x lo-'; H3Si0,, 1.26 x lo-"; H,SiO;-, 2.41 x H,Si04, 7.75 x lo-''; H S ~ O ~2.70 - , x lo-,, the neutral complex CaH,SiO: thus being the principal Si0,-containing species. The concentrations of Ca2+,CaOH+ and OHare not significantly affected. 5.5.3 Thermochemistry and thermodynamics
Using a heat of solution method, Lerch and Bogue (L24) found the enthalpies of hydration of C,S and P-C,S in pastes to be - 115 kJmol-' and -45 kJ mol-', respectively. Similar values have been reported by other investigators, e.g. - 1 18 kJ mol- ' and -45 kJ mol- ' by Brisi (B60) and - 121 kJ mol- and -46 kJ mol- by Fujii and Kondo (F18), who recalculated data obtained by Brunauer et 01. (B61) for dried products. Brunauer ct ul. noted that, if the hydration products of C3S and P-C,S were identical apart from the quantities of CH formed, the enthalpy of hydration of C,S should equal the sum of the enthalpies of hydration of P-C2S, hydration of CaO (AH,,, = -65.2 kJ mol- ') (W lo), and reaction of P-C,S and CaO to give C3S (AH,,, = - 13.4 kJ mol-') (WIO). For the moist material, this sum is - 124 kJ mol-', which is near the observed value. Fujii and Kondo (F18) calculated the thermodynamic properties of C-S-H, using solubility data and properties of 1.4-nm tobermorite and CH given by Babushkin et al. (B62). The C-S-H was treated as equivalent to a solid solution of these two compounds. For the composition 1.7Ca0.Si02.2.617H20, they found AHf" = -2890 kJ mol- ', AGf0 = -2630 kJ mol-' and S" = 200 J mol-' K - ' . They showed that these values were consistent with data for the enthalpies of hydration of C3S and P-C,S. The free energy of formation of C-S-H from 1.4-nm tobermorite and CH was shown to become increasingly negative with increasing Ca/Si ratio, but to tend towards a limit at ratios approaching 2.0; this is consistent with the observed upper value of this quantity.
'
'
158
Cement Chemistry
Glasser rt rrl. ((350) calculated the activities of the principal ionic species in solution in equilibrium with C-S-H of various Ca/Si ratios, and solubility products defined in terms of those species. Using values of AG, ' for the ionic species (B62). values of AG," for the solids were then calculated. The results depended somewhat on the set of solubility data used, but were broadly similar to those obtained by Fujii and Kondo (FI 8). Using one set of data, values ranged from - 1720 kJ mol-' at Ca/Si = 0.93 to - 2350 kJ mol- ' at Ca/Si = 1.63, referred to formulae containing one silicon atom in each case. The Gibbs-Duheni equation has been used to calculate Ca/Si ratios of solids from the CaO and SiO, concentrations in solutions with which those solids are in equilibrium (F19,G48). Gartner and Jennings ((348) showed that, at the low concentrations involved, this equation reduces to the approximate form (Ca/Si),,,,, = -dps/dpc, where p, and p, are the chemical potentials of SiO, and CaO, respectively. The calculations were carried out for curve B and three variants of curve A on Fig. 5.12. For points along each curve, activities of the ionic species in solution were calculated, and from these, values of pS and p,; these were plotted against each other to give curves on a chemical potential phase diagram. From the slopes at points along these curves, the Ca/Si ratios of the solids were obtained. For one variant of curve A, the calculated Ca/Si ratios at C,,, = 3 mniol I - ' and 14.4 mmol I were 1.0 and 1.45 respectively. These values agree well with typical experimental results shown in Fig. 5.1 I . For curve B, the calculated Ca/Si ratios were I. I at 5 mmol 1 - I , 1.65 at 13 mniol l - ' and 1.65 at 22mmol I - ' .
-'
5.5.4
Effects of alkalis
Several studies on the quaternary systems of Ca0-Si0,-H20 with Na,O or K 2 0 have been reported (K19,S52,M52). Alkali greatly lowers the concentrations of CaO in the solution and raises those of S O 2 . The solid phase compositions are difficult to study. Determinations based on changes in concentration on adding CH to alkali silicate solutions are subject to considerable experimental errors, while direct analyses of the solid are difficult to interpret because the alkali cations are easily removed by washing. Suzuki et a/. (S52) considered that they were adsorbed. Macphee et ( I / . (M52) reported TEM analyses of the C-S-H in washed preparations obtained by reaction of C,S (10 g) in water or NaOH solutions (250 ml). The C-S-H obtained with water had a mean Ca/Si ratio of 1.77; that obtained with 0.8 M NaOH had a mean Ca/Si ratio of 1.5 and a mean Na,O/SiO, ratio of 0.5. These results do not appear to be directly relevant to cement pastes. The pore solutions of the latter may be 0.8 M or even higher in alkali
Hydration of the Calcium Silicate Phases
hydroxides (Section 7.5.2), but the ratios of ( N a 2 0 C-S-H are 0.05 or below (Section 7.2.5).
+ K 2 0 ) to SiO,
159
in the
5.6 Kinetics and mechanisms
5.6.1
C,S: experimental data
Fig. 5.13 shows the general form of the curve relating the fraction of C,S consumed (a) to time in a paste of w/s 0.5 at about 25•‹Cand with moist curing. Such curves have been determined using Q X D A for unreacted C,S (e.g. Refs K20,010), though the precision is low for values of a below about 0. I. At low values of a , other methods are available, such as conduction calorimetry (e.g. Ref. P22), aqueous phase analyses (e.g. Ref. B63) or determinations of C H content or of non-evaporable water. At very early ages, it may be necessary to allow for the fact that the property determined depends on the nature of the hydration products; e.g. precipitation of C-SH begins before that of CH. I. T i m e
(days)
2. T i m e
(hours)
Fig. 5.13 General form of curves relating the fraction of C,S consumed to time in a paste. AB: initial reaction. BC: induction period. CD: acceleratory period. DE: deceleratory period and continuing, slow reaction. Taylor rt crl. (T25).
160
Cement Chemistry
The rate, as followed from that of heat evolution (Fig. 5.14), passes through an initial maximum, decreases to a minimum during a so-called induction period, passes through a second maximum and then gradually declines. Kondo and Ueda (K20) described five periods of reaction, viz: (1) the initial reaction, (2) the induction period, (3) the acceleratory period, in which the main reaction first begins to occur rapidly, (4) the deceleratory period and (5) a period of slow, continued reaction. Periods 1 and 2 correspond to the early stage of reaction as defined from n~icrostructural studies (Section 5.3.1), periods 3 and 4 to the middle stage, and period 5 to the late stage. Setting takes place during the acceleratory penod.
Ti m e (hours) Fig. 5.14 Typical calorimetric curve relating the rate of heat liberation to time in a C,S paste to time. After Brown el ul. (B64).
Many investigators have followed the changes in C,,, and C,,02 or [OH-] with time in the course of the reaction of water with C,S or p-C,S in suspensions (e.g. B63,B56,G51; Fig. 5.15). The curves for C,, or [OH-] typically show a steep rise during the initial reaction and a more gradual one of uniform slope during the induction period. Significant precipitation of CH begins just before the maxima are reached, and is rapid when they decrease, during the acceleratory period (W22). Some of the curves reported in the literature show a plateau in the early part of the induction period, followed by a rise (e.g. Ref. B57). In the most marked cases of this behaviour, the maximum is unusually high ( - 40 mmol 1 - ') and unusually delayed (24-36 h; e.g. Ref. F20). As noted in Section 5.5.1, the final values reported for pastes tend to be well above the solubility of CH. The SiO,
Hydration of the Calcium Silicate Phases
161
concentration passes through a maximum in the order of 1 mmol I - ' during the initial reaction. Brown rt al. (B56) found the concentrations to lie exclusively on curve A (Fig. 5.12), but other workers have found the paths followed by the concentrations to cross curve A, passing through maxima near to (G51) or beyond (B57,B58) curve B and then falling back to curve A. At early ages, daldt increases markedly with w/s ratio above 0.7 (B56). Moderate variations in specific surface area have little effect on the length of the induction period, but with finer grinding, daldt during the acceleratory period increases (K20,012,B56). The rate of reaction increases with teniperature up to the end of the acceleratory period, but is much less affected thereafter (K21), suggesting a change from chemical to diffusion control. Introduction of defects into the C3S shortens the induction period (M53,F20,012).
Fig. 5.15 Plots of the analytical concentrations of CaO and SiO, against time for a C,S paste (C,S 352.5 m2 k g - ' , w/s = 0.7, 24•‹C).After Brown ct 01. (B56).
The results of experiments at high w/s ratios have been interpreted assuming superficial hydroxylation of the C3S followed by congruent dissolution and subsequent precipitation (B63,B57). Reaction in pastes must also occur at least partly by dissolution and precipitation, since much of the product is deposited at a distance from the starting material, and because both C,, and Csio, pass through maxima. Topochemical mechanisms. defined as ones in which the material does not pass through a true solution phase, may also occur, especially in the late stage of reaction (T26). The kinetics up to the middle of the acceleratory period are discussed in the following sections. Those of the later stages have been more thoroughly studied with cement (Section 7.7) and only some aspects are considered here.
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Cement Chemistry
5.6.2 C,S: the initial reaction Two ESCA studies of the hydration of C,S pastes or compacts showed an initial surface Ca/Si ratio of 3.0, which within the first minute fell sharply and then increased again; it then fell more slowly, reaching a value of 1.5-2.0 in 15-30 min (T27,M39,R18). Allowing for the fact that the region analysed will include unreacted C,S until the layer of hydration product is sutticiently thick and continuous, these results may be interpreted in terms of the formation of an initial hydrate of low Ca/Si ratio, which rapidly increases (R 18). Another study (B65), in contrast, showed an unreacted C,S sample to have a low surface Ca/Si ratio. ESCA results also indicate a change in the environment of the silicon atoms at the end of the induction period. possibly associated with the formation of dinieric silicate ions (R 18). Secondary ion mass spectrometry (SIMS) shows promise as an alternative method for studying surface compositions, but has not yet given information on that of the hydrated material (G52). Electron diffraction shows that an amorphous surface layer can be produced by grinding (UIO,K20); this may explain some of the reported differences in behaviour between apparently similar specimens. Calculations of a from concentrations in the bulk solution indicate values of around 0.3% at the start of the induction period (B63.BS6). These are likely to be minimum estimates, because they assume that no CH is formed; in reality, small amounts may be, on account of concentration gradients, which will themselves introduce an error in the same direction. Reported values of a at the end of the induction period, summarized by Odler and Dorr (OIO), range from 0. I % to an improbable 8%. Their o\vn estimate of 1-2% is supported by subsequent evidence from cross-polarization 29Si NMR (R 16). Assuming a specific surface area of 300 mZkg- '. a value of 1% corresponds to a mean depth of attack of about 10 nni. 5.6.3 C,S: the induction period The causes of the induction period and of its termination ha\e been the subject of much debate. Hypotheses have been reviewed in a collaborative paper (T25). The main ones are as follows: (1) The product of the initial reaction forms a protective layer on the C,S particles; the induction period ends when this is destroyed or rendered more permeable by ageing or phase transformation (S53,B66.J15). (2) The product of the initial reaction forms a semipermeable membrane which encloses an inner solution and is destroyed by osmotlc bursting (Dl 8).
Hydration of the Calcium Silicate Phases
163
(3) The rate of reaction in the induction and acceleratory periods is controlled by nucleation and growth of the C-S-H formed in the main reaction, the induction period ending when growth begins (S54,013,F20,B63). (4) The induction period occurs because the CH nuclei are poisoned by SiO, and cannot grow, and ends when the level of supersaturation is sufficient to overcome this effect (G53,T28,W22). The induction period is shortened by adding prehydrated C,S (013), but additions of lime or CH, including that formed from C3S, are variously reported to be ineffective (013,B67) or to lengthen it, though shortening it with cement (U11). In cement mixes, additions of pfa or some other finely divided materials accelerate hydration after the first day, apparently by acting as nucleation sites for C-S-H (Section 9.3.3). Additions of reactive silica markedly accelerate hydration (S53). Most of this evidence supports hypothesis 3 and tells against hypothesis 4. Hypothesis 3 does not exclude hypothesis 1, as the breakdown of a protective layer could be associated with formation of a new product. The recent microstructural evidence (Section 5.3.1) gives no indication that a membrane or other product distinct from that formed later is formed during the initial reaction in C3S pastes, though a gelatinous coating is formed in cement pastes, which show an induction period similar to that observed with C,S (Section 7.5.1). For C3S pastes, this evidence excludes hypothesis 2, and gives no positive support to hypothesis 1. It does not exclude the formation of an altered layer on the C3S surface, no more than a few nanometres thick. Tadros et al. (T28) postulated the formation of a Si02-rich layer with chemisorbed C a 2 + , and Barret et al. (B63) that of a superficially hydroxylated C3S, formed by protonation of the 0'- and Si0;- ions, balanced by loss of C a 2 + . The calculated solubility of C3S is about I molal (S50). In the absence of an altered or protective layer, the observed low rate of reaction during the induction period could be explained only if the concentrations close to the C,S surface were of this order of magnitude. It is doubtful whether the very high concentration gradients that this implies could exist (J15,G49). The balance of the evidence favours a combination of hypotheses 1 and 3. The following model is essentially that of Gartner and Gaidis (G49), but has features in common with those of Barret and Bertrandie (B57) and of Grutzeck and P amachandran (G5 1). (1) In the initiai reaction, C,S dissolves and a material is deposited which will be termed Product B. The dissolving C,S probably has a hydroxylated surface, as proposed by Barret et al. (B63). Under typical paste conditions, Product B forms an overlying surface layer about 1 nm in
164
Cement Chemistry
average thickness, though it is likely to be non-uniform and thicker around active sites. It may be capable of existence only on a C,S surface and similar to a passive layer on a metal. Within 30 s, the C,S is almost isolated from the solution, and an unstable equilibrium, represented by curve B of Fig. 5.12, is established between the solution and Product B. (2) Product B is unstable with respect to the material represented by curve A of Fig. 5.12, for which we shall reserve the term C-S-H. During the early part of the induction period, C-S-H nucleates and begins to grow. This implies dissolution of Product B, and increased access of solution to the C,S, which thus dissolves more rapidly. The rate of reaction in this stage is controlled by the growth of C-S-H, the rate of which increases with the amount already formed. The amount of Product B at any given stage in the reaction must depend on the relative rates of its formation and dissolution. It may or may not disappear completely when the latter is sufficiently high. The NMR evidence of Rodger et al. (R16) (Section 5.3.2) indicates that the hydrated material formed up to the end of the induction period contains only monomeric silicate ions and that the latter persist alongside larger ions when the latter are formed. This suggests that the silicate ions are monomeric both in Product B and in C-S-H in the early stages of formation of the latter product. The Ca/Si ratio of Product B is possibly about 1 initially, increasing to about 1.7 as equilibrium with CH is approached.
5.6.4
The main reaction (C,S and p-C,S)
From the kinetic standpoint, there is probably no distinction between the induction period and the early part of the acceleratory period; indeed, as Gartner and Gaidis (G49) noted, there is strictly no induction period in the absence of retarding admixtures. Throughout both stages, growth of C-S-H proceeds at an increasing rate, which is reflected in increasingly rapid dissolution of the C,S. The C a 2 + and OH- concentrations in the bulk solution increase steadily, and when a sufficient degree of supersaturation has been reached, CH begins to precipitate in quantity, causing these concentrations to fall. Its formation is not normally rate determinmg, but could become so in the presence of admixtures that retard its growth. The kinetic data for this stage can be fitted by Avrami-type equations, but the assumptions underlying the latter (formation of a product within an initially homogeneous material) do not correspond to this situation existing in the hydration of C,S or cement.
Hydration of the Calcium Silicate Phases
165
The smaller grains of C,S probably react completely, by dissolution and precipitation, during the acceleratory period. In the final, slow periods of hydration, the remaining grains of C,S, which are relatively large, are g-radually replaced by C-S-H through the inward movement of an interface. It is difficult to believe that such a process occurs by dissolution into a true liquid phase, followed by precipitation. Following earlier studies (M49.F21,K22,K20), Taylor (T26) suggested that a topochemical reaction may occur with both C,S and 0-C2S. Comparison of the numbers of atoms of each element in the initial and product phases shows that to convert either C,S or P-C,S into C-S-H, relatively little oxygen must be gained or lost; much of the calcium, and smaller proportions of the silicon, must be lost. and equivalent amounts of H + gained (Table 5.6). At the interface, a narrow zone could exist, in which the necessary atomic rearrangements take place. I t is not necessary to postulate any migration of water molecules through the inner product; instead, Ca2+ and Si4+ move outwards, and H" inwards. At the surface, the calcium and silicon enter the solution, and ultimately precipitate, along with the O H - ions released from the H 2 0 molecules, as CH and outer product C-S-H. Migration of silicon was suggested as the probable rate-determining step; however, at low w/s ratios, reaction stops because there is no more space in which the outer product can be formed, and in this case, control presumably shifts to some process related to deposition of outer product. Barret (B68) discussed topochemical mechanisms and their relation to dissolution-precipitation processes. Table 5.6 Numbers of atoms in IOOO/N, ml
Phase
Density (kg m - 3,
c3s
3 120 3326 1900
P-Cls
C-S-H (90% RH)
Ca
Si
H
0
Slightly modified from Ref. T26: C-S-H composition assumed C,,,,SH,,,
5.6.5
Early hydration of 0-C,S
The kinetics and mechanism of P-C2S hydration are similar to those for C,S. apart from the much slower rate of reaction (M53,T29,F1 I,014,F22), and, as noted earlier, the products are similar apart from the much smaller content of CH. Preparations appear to be more variable in reactivity than those of C,S; this is partly attributable to differing stabilizers (F23), but could also be due to differing amounts or natures of phases in intergranular spaces or exsolution lamellae. Preparations made at low temperatures are
166
Cement Chemistry
especially reactive (F22). Because of the low rate of heat evolution, conduction calorimetry is difficult, but shows the existence of an induction period similar to that found with C,S (M53), which is also indicated by QXDA (014). ESCA, SEM and solution studies show that behaviour during the early period is similar in principle to that of C,S (M54). The ESCA data show that the connectivity of the silicate tetrahedra increases within 24 h at 50•‹C; this behaviour resembles that of C,S at the end of the induction period. They show the approximate depth of the hydrated layer to be 5 nm after I5 s, and 15 nm after 6 h at 25•‹C.The rate of reaction is increased in the presence of C,S (T29,014).
Hydrated aluminate, ferrite and sulphate phases
6.1 AFm phases
6.1 .I
Compositional and structural principles
AFm (A1203-Fe203-mono) phases are formed when the ions they contain are brought together in appropriate concentrations in aqueous systems at room temperature. Some can also be formed hydrothermally. i.e. in the presence of water under pressure above 100•‹C.They are among the hydration products of Portland cements. Under favourable conditions. they form platey, hexagonal crystals with excellent (0001) cleavage. Some of the AFm phase formed in Portland cement pastes is of this type, but much is poorly crystalline and intimately mixed with C-S-H. AFm phases have the general formula [Ca,(AI,Fe)(OH),].X.uH,O, where X denotes one formula unit of a singly charged anion, or half a formula unit of a doubly charged anion. The term 'mono' relates to the single formula unit of CaX, in another way of writing the formula, viz. C,(A,F)CaX2.yH20[or C,(A,F)X,.j.H20]. where y = 2(x + 3). Many different anions can serve as X, of which the most important for Portland cement hydration are O H - , SO:- and COi-. A crystal may contain more than one species of X anion. AFm-type phases can also be prepared in which other tripositive cations, such as Cr3+. replace the A13 or Fe3 . AFm phases have layer structures derived from that of CH by the ordered replacement of one Ca2+ ion in three by AI3+ or Fe3+ (Fig. 6.1 ) (A7-A9). The principal layers thus defined alternate with interlayers containing the X anions, which balance the charge, and H 2 0 molecules. The replacement of Ca2+ by the smaller AI3+ or Fe3+ ions distorts the structure of the principal layer, alternate Ca2+ ions moving in opposite directions from its central plane. This allows each to coordinate the oxygen atom of an interlayer H 2 0 molecule in addition to its six O H - ions. The principal layer, together with the H 2 0 molecules thus bonded to the C a 2 + ions, has the composition [Ca2(Al,Fe)(OH),.2H,0]+. In the simpler AFm structures, these units are stacked in such a way as to produce octahedral cavities surrounded by three H,O molecules from each of the adjacent layers. These cavities may contain +
+
168
Cement Chemistry
X anions, H,O molecules, or both. In C,ACaC12.10H20 (or [Ca,AI(OH), .2H20]CI), each cavity contains a Cl- ion. In C3ACaSOi 1 2H20, one half contain SO:- anions and the remainder contain two H 2 0 molecules. In C,AH,,, all contain one O H - anion and one H 2 0 molecule.
Fig. 6.1 Structure of a single principal layer of composition [Ca,AI(OH),]' in an AFm phase, in ab-projection. Distances of calcium. aluminium and oxygen atoms above or below the central plane of the layer are in picometres. Hydrogen atoms are not shown. Large circles illustrate the distortion of the CaO, octahedra. which allows the coordination of each calcium atom to rise to 7 through the addition of an H,O molecule (not shown) directly above o r below i t in this projection.
The unit cells of all AFm phases are based on hexagonal structural elements with a = 0.57-0.59 nm. These values are somewhat less than J3 times that of CH. The layer thickness, c', depends on the nature of the X anion and the amount of interlayer water, which can be varied by stepwise dehydration from the highest hydration state. AFm phases exist in which a complete, additional layer of H 2 0 molecules is present between the principal layers. There is no restriction on the separation between adjacent layers; this allows large anions, and also intercalated neutral molecules (D19,D20), to be present in interlayer sites. Many AFm phases readily undergo changes in water content and anion exchange (F24,D20). the latter including that of COi- for OH-. Due care must therefore often be taken to exclude CO,, and where necessary also to control the humidity. Complex sequences of stacking the layers, or ordered patterns of filling of the cavities where their contents are not all the same, lead to unit cells that 1
,
Hydrated Aluminate, Ferrite and Sulphate Phases
169
are larger or of lower symmetry than the structural element. Some AFm phases show polytypism, arising from different ways of stacking the layers.
6.1.2
The C4AHx,c,AC,,,H,
and
C,ACH,
phases
Fischer and Kuzel (F25) summarized and extended earlier work on these groups of AFm phases, which are listed in Table 6.1. In the pure CaOAI,O,-H,O system, the phase in equilibrium with solutions of appropriate composition at temperatures up to at least 50•‹Cis C4AH,,, of which two polytypes ( a , and a,) are known. On decreasing the RH or increasing the temperature, lower hydrates are reversibly formed, with decrease in layer thickness; a decrease in RH to 8 1 % is sufficient to produce C4AH,, (R 19). C,AH,, is structurally derived from C,AH,, by the addition of an extra layer of H 2 0 molecules. A high degree of crystallinity is needed to allow this extra layer to form (D21), and it is doubtful whether it could occur in the material in a cement paste. In C4AH,,, the contents of the octahedral cavities in the interlayers are reduced from (OH- + H,O) to OH-, and in C4AH,, which is a poorly crystalline material, the H,O molecules attached to the Ca2+ ions have also been lost. The layer thicknesses indicate that the C 0 5 - ions are oriented perpendicular to the layers in c,Ac,,,H,, and parallel to them in the other phases containing COi-. In all these phases, some octahedral cavities contain a C04- ion, and others varying combinations of H 2 0 molecules and OHions. In the early literature, c,ACH,, (or C,A.CC.H, ,) was incorrectly identified as a C,A hydrate, and c~Ac,,,H,, as a polymorph of C,AH,,. Both phases readily form on carbonation of the C4A hydrates. Table 6.2 includes crystal data for the C,AH,, C,AC,.,H, and C,ACH, groups. Only data based on single crystal studies are included. Solid solutions in these groups of phases appear to be very limited. only c,AC,.,H,, showing some variation in layer thickness up to a maximum of about 0.825 nm associated with partial replacement of C 0 3 - by 2 0 H together with variations in H 2 0 content (D21,F25). The limiting composition is c,AC,,~,H,. More extensive solid solution may occur in the poorly crystalline material formed in cement pastes.
6.1.3 The
C,ASH, phases
Tables 6.2 and 6.3 give data for these phases. C,ASH,, (or C,A.CS.H,,) is variously known as monosulphate, monosulphoaluminate, or, in the early literature, low-sulphate calcium sulphoaluminate. The water contents of the
Table 6.2 Crystal data for AFm phases
Phase ~I-C~AHI~ 19
C4AH I 3 C4AC0.5H,2
C,A~H,, C,A~H, a-C,ACaCI,.H P-C,ACaCI,.H,, C,ACaCI,.H,
,
C2AH7sC
C2AH5 C,ASH,
,,
Unit cell parameters
Crystal system"
Space group
H H H H T H M H H M H H
R ~ or C R3c ( ? ) P6,/m or P6,22 R-R3c or R3c PT or P1 R3 C2/c or Cc R3c or R3c R3c or R3c C2/c or Cc R ~ C ~3 or R3
b(nm)
c(nm)
Angles
0.577 0.577 0.5752 0.5770 0.5781 0.576 0.998 0.574 0.5729
-
6.408 2.137 9.527 4.9159 0.7855 2.679 1.679 4.688 4.1233 4.22 5.22 3.764
y = 120" y=120•‹ y = 120" y=120•‹
= hexagonal or trigonal; M = monoclinic: ' a = 92.661'. P = 101.96". y = 120.09.
T
=
-
0.5744 -
0.574 -
-
0.993
0.574
0.573 0.5747
-
-~
"H
Dx
a(nm)
-
Z
3 1 6 3 b 112 y = 120" 312 /3 = 110.2" 2 y = 120" 3 y=12OC 3 /3 = 97.0" 8 y = 120" 6 y=120•‹ 3 -
-
-
-
-
-
-
(kg m-,)
Ref.
1803 1802 2046 1984 2170 2014 2065 2090 2080 1943 2042 1936
A10 A10 F25 F25 F25 A1 1 K23 K23 K24 S55 S55 K25
-
-
triclinic.
For C,AH,. only the structural element (u = 0.574 nm, c
=
1.07 nm. Z
=
I ) and X-ray density (1950 kgm-') are known 6 5 5 ) .
Table 6.3 AFm phases with [Ca,Al(OH),]+ principal layers and interlayer sulphate, chloride, aluminate or aluminosilicate anions Composition
Drying conditions
C,ASH, 6 b C,ASH, ,b c,ASH,, C,ASH, C,ASH,
< 10•‹Cat 100% RH > 10•‹Cat 100% RH > 10•‹Cat 20-95% RH > 10•‹Cat < 20% RH 30-50•‹Cover P 2 0 5
C3ACaC12.H,, (a) C,ACaCl2.Hl0 (P) C3ACaC12.H6
< 2 8 T at 35% RH > 2 8 T at 35% RH 120-200'C
C2AH8 C2AH7.5
< 26'C at 45% RH > 2 6 T at 45% RH Room temp. over P 2 0 ,
C2ASH, C2ASH4
Room temp. at 37% RH > 135-C
" Per formula unit of Ca,[AI(OH),]+. Water contents uncertain; see text. ' Al is 6 coordinated in C,AH, ((354).
Layer thickness (nm)
Interlayer contents4
Ref.
Hydrated Aluminate, Ferrite and Sulphate Phases
173
two highest hydrates are uncertain, values of 16 for the 0.96-nm hydrate (T30) and of 15 for the 1.03-nm hydrate (R19) having also been reported. An alternative formula with 18 H 2 0 (M55) for the 0.96-nm hydrate implies an impossibly high density for the additional water. As with C4AH19, these highly hydrated forms probably cannot occur in the poorly crystalline material of cement pastes. c,ASH,, forms solid solutions in which up to one half of the is replaced by OH-. The substitution does not significantly affect the length of the u axis, but c' is slightly reduced, to a value of 0.877 nm at maximum replacement (R19). The higher hydrates do not form if excess CH is present. Early reports that a continuous solid solution series exists with C,AH, are incorrect. They were based on observations by light microscopy, which with layer structures of this type cannot distinguish between a true solid solution, in which the components are mixed at or below a nanometre level within each layer, an oriented intergrowth consisting of blocks each containing many layers of one or other component, or intermediate possibilities, such as random interstratification of layers of different types. Distinction can be made by XRD. A phase of approximate composition c,A,,,N,,,s,, H was reported to form at high concentrations of Na,O, AI2O3 and SO, (D22). Three hydration states, with c' = 1.00, 0.93, and 0.81 nm, and 16, 12 and 8 moles of H 2 0 , respectively, were described, and it was suggested that the structure was derived from that of C,ASH, by the omission of some of the A13+ ions, balanced by the inclusion of Na', in addition to anions, in the interlayer. The concentrations used in the original work were far removed from those existing in the liquid phase of cement pastes, but later work (S56) indicated that a similar phase, though probably of different composition, could form in highly alkaline media in the presence of excess CH and absence of excess A120,. This suggests that it could occur as a hydration product of cements high in alkali. Evidence that AFm phases can accommodate cations as well as anions in the interlayer is provided by the existence of a compound [Ca2AI(OH)6]K~(CI04)2~xH20 (D20).
, ,,
6.1.4
Other AFm phases containing aluminium
The range of anions that can occupy the X positions is very wide (A1 I,D20); even anions that form calcium salts of very low solubility, such as fluoride, can be introduced using special techniques. We shall consider only those most relevant to cement chemistry. Tables 6.2 and 6.3 include data. C3ACaC1,~10H,0 or [Ca2AI(OH)6]C1~2H20 (Friedel's salt) can be formed in concrete exposed to chloride solutions. Two polytypes are known; P is the
174
Cement Chemistry
higher temperature form, produced reversibly from a at 28•‹C. C3ACaC12~10H20 does not form solid solutions with the sulphate AFm phases. but an ordered compound c , A , . c s C ~ C I , . ~ ~ H , O exists, in which interlayers containing CI- probably alternate with ones containing SO:(K23). The C,A hydrates are AFm phases with aluminium-containing species in the interlayer. The hydrate formed in metastable equilibrium with aqueous solution at 18•‹Cis C2AH8.Scheller and Kuzel (S55) found that, contrary to some earlier opinions, C2AH, shows neither polymorphism nor polytypism, but that below 45% RH at 26•‹Cit is converted into C,AH,,,. The transition temperature depends on the RH. An X-ray structure determination on the lower hydrate, C2AH,, showed that this had the constitution [Ca2AI(OH),][Al(OH),], the interlayer aluminium and some of the interlayer hydroxyl groups being statistically distributed. Because of poor crystallinity, i t was not possible to obtain such detailed information about the higher hydrates, but some evidence was obtained for a similar constitution, with H,O molecules added, in C2AH,,,. In contrast, a study of C2AH8 us;qg "AI magic angle N M R showed that all the aluminium was octahedrally coordinated (G54), and the constitution [Ca2AI(OH),][AI(OH)3(HZO)3]OHwas proposed. The structural element of C,AH, is closely similar to that of C,AH,,, and the proposed constitution could be derived from that of the latter compound merely by removing some of the hydrogen atoms from interlayer H 2 0 molecules and introducing aluminium atoms, perhaps statistically, into octahedral sites. There is no necessary conflict between the two sets of data, as they relate to different hydrates. C2AH8is reported to form solid solutions with C,AH,,, but there is disagreement as to the end of the series at which these occur (J 17,D20). An early report of the formation of C3ACS.H, was not supported by later work (C28), but three subsequent investigations have shown that such a phase can be prepared, though it is unstable at ordinary temperatures. and possibly so even in contact with its mother liquor at 5•‹C. The anion is 1.01 nm and 0.89 nm, probably H,Si042-, and hydration states with c' presumably analogous to those of C,ASH,, have been observed. I t was obtained by anion exchange (D20), in reaction rims around C,A grains in pastes made from C,A, C,S and gypsum (R20) and by reaction between C,AH,, and amorphous silica in the presence of water at 5•‹C(V4). Because of its instability, it is unlikely to be more than a transient hydration product of cement, at least at ordinary temperatures, but the possibility of partial replacement of SO,'- by H,SiO,'- in an AFm phase cannot be exclude C2ASH,, known by its mineral name of stratlingite and also as 'gehleni hydrate', is well established as a natural mineral, hydration product certain types of composite cements and laboratory product. Its crystal da
--
Hydrated Aluminate, Ferrite and Sulphate Phases
175
(Table 6.2) show it to be an AFm phase having an aluminosilicate anion as interlayer and probable constitution [Ca2AI(OH),][AISi0,(OH),.4Hfl] (K25). Naturally occurring AFm phases, called hydrocalumite, appear to vary in composition. The original specimen was reported to have a conlposition near to C,AH12 with some C O i - and a monoclinic unit cell similar to that of a-C,ACaC12~10H,0(T31), but others have proved to be either the a or the polymorph of that compound or members of a C3ACaC12.10H20C,AH,, solid solution series (F26).
6.1.5 AFm phases containing iron
Iron(II1) analogues of many of the phases described above have been obtained. Unless the anion imparts colour, AFm phases containing ~ e "are colourless, brown colours being due to iron(II1) oxide or hydroxide impurity. They have a-axial lengths of approximately 0.589 nm (S57,K26). C,FH,, has been obtained by many investigators (e.g. S57,R21). It has L" = 0.79nm and forms a continuous series of solid solutions with C,AH,, (S57). As in the corresponding system with A1,03, the phase existing in contact with solution is the 19-hydrate (R21). C,FSH, phases are also well established (M56). Kuzel (K26) found that c,FSH,, is isostructural with its A13+ analogue, and has c' = 0.8875 nm; a higher hydrate with c' = 1.021 nm and probably containing 14 H 2 0 exists above 9o0h RH. c,FSH,, and C,ASH,, form a continuous solid solution series at 100 C. but at 25•‹Cor 50•‹C,miscibility is incomplete. At 2 5 T , c,FSH,, accomrnodates up to about 50 mole % C,ASH,,, and the latter up to about 10 mole % C,FSH,,, and an intermediate phase with Fe/AI = 0.5 exists (K36). C,F~CaCI,~IOH,O similarly shows only limited miscibility with C,ACaC12~IOH,0 (K26). Schwiete et al. (S58) found no solid solution between C,FH,, and C,FSH,,, but in preparations made from C,AF or C,A,F limited miscibility occurred at the sulphate-rich end of the series as with C,AH,, and C,ASH,,. Fe3+ analogues of the A13+ phases containing C0,,- appear to exist (R22,S56).
6.1.6 XRD patterns, thermal behaviour, optical properties and IR spectra
Strong peaks in the XRD powder patterns of AFm phases normally include the first and second orders of the layer thickness, and the 1120 and 3030
176
Cement Chemistry
reflections, which for the A13+ compounds have d-spacings at or near 0.2885 nm and 0.1666 nm, respectively. In mixtures, it is sometimes difficult to distinguish between AFm phases of similar layer thickness, and in such cases, examination of the patterns of heated samples is often useful. I
I
I
6
I
I
TEMPERATURE
1
(OC
-
500 700 TEMPERATURE (OC) Fig. 6.2 Weight loss curves for C,AH,, (static; Buttler er (11. (B69))and monosulphate and ettringite (TG; 10 deg C min- '; Taylor (TI6)). 100
300
Weight loss curves under either static or dynamic (TG) conditions or both have been reported for C,AH,, (B69,L6), c,ACH,, (T32,F25), c,AC,,,H,~ (F25), C , A ~ H , , (T16), C,A~CaC12~IOH20 and other phases (A9,K24), C2AH, (L6) and C2ASH, (K25). The curves for C,AH,, and C,ASH,, (Fig. 6.2) and of C2AH, are all fairly similar if the quantity plotted against temperature is moles of H 2 0 retained per mole of CaO, and allowance is made for the effect of differing heating rates. With the carbonate, nitrate and bromide, and on static heating also with the chloride, volatiles in addition to H 2 0 are lost. All the curves show steps in varying degrees. Those at the lower temperatures are associated with the changes in hydration state of the AFm phase; loss of molecular water is complete by
Hydrated Aluminate, Ferrite and Sulphate Phases
1 77
about 150•‹C on static, or 250•‹C on dynamic heating. X R D evidence showed that, for C4AH13, the plateau at about 350•‹C corresponds to formation of a poorly crystalline mixture of CH and C4A3H,, and the steps at about 410•‹C and 630•‹C to decomposition of these phases to CaO and C12A,H plus CaO, respectively (B69). The curve for c,AsH,, shows similar features, but their significance has not been determined. The DTA curves of C,AH and c,ASH,, are characterized by a series of endotherms below 300•‹C. which correspond to the steps in the TG curve (K27,M57). Fig. 6.3 includes a typical curve for c,AsH,,; that of C4AH,, is similar, but with peak temperatures some IOdegC lower. The peak temperatures depend on the tech~~ique and amount present, the 200•‹Cpeak being typically shifted to 180-190•‹C for the material present in cement pastes (B70). DTA curves have been reported for other AFm phases, including C,ACH,, (T32), C3ACaCI,.IOH20 (A9,K24) and C2ASH, (K25).
,,
MONOSULPHATE
200
ETTRlNGlTE I
I
I
1
I
1
I
I
I
700 9 00 T EMPERATURE ( " C ) Fig. 6.3 DTA curves for monosulphatc and cttringite (IOdegC m i n - ' ) . Taylor 100
300
500
(T5). As would be expected from their layer structures, AFm phases are optically negative. The A13+ phases typically have o = 1.50-1.56. E = 1.491.54; for the Fe3 phases, o is typically 1.56-1.61 and E is 1.54-1.60. Henning (H34) reviewed IR spectra of hydrated calcium silicates and aluminates; data for C,AH, and C2AH, phases, C,ASH,,, c,ACH,, and c,FSH,, were included. Other data for phases in the CaO-A1203-H,O system (B71) and for c,ASH,, (B72) have been reported. +
6.2 AFt phases
6.2.1
Compositions and crystal structures
AFt (A1203-Fe20,-tri) phases have the general constitutional formula
178
Cement Chemistry
[ C a , ( A I , F e ) ( O H ) 6 ~ 1 2 H 2 0 ] 2 ~ X H ,where 0, x- < 2 and X represents one formula unit of a doubly charged, or, with reservations, two forn~ulaunits of a singly charged, anion. The term AFt refers to the three units of CX in an alternative way of writing the formula, C3(A,F).3CX.yH,0 [or C,(A,F)X,.J~H,O], where JJ = .u + 30. AFt phases are formed under broadly similar conditions to AFm phases, but at higher ratios of CaX to C3(A,F) and rarely above about 90•‹C.The range of anions that can occupy the X sites is smaller, and singly charged anions can possibly only be accommodated to a limited extent. The most important AFt phase is ettringite, [Ca,AI(OH),.12H20]2.(S0,)3.2H20 or C3A.3CaSO4.32H20; a phase of or near this composition is formed during the early hydration of most Portland cements. In the early literature, it was often called high-sulphate calciun~sulphoaluminate. Ettringite also occurs as a natural mineral. Phases of AFt type also exist in which other cations replace the Ca2 or A13+ or both; an important example is thaumasite, Ca,[Si(OH),. 1 2H20](C03)(S04). AFt phases form hexagonal prismatic or acicular crystals. Their structures (C29,M58,M59) are based on columns in hexagonal array. running parallel to the prism (c) axis, with the X anions and, usually, H 2 0 molecules in the intervening channels (Fig. 6.4). The columns, which are of empirical are composed of (AI.Fe)(OH), octaformula [Ca3(AI,Fe)(OH)6.12H20]3+, hedra alternating with triangular groups of edge-sharing CaO, polyhedra, with which they share O H - ions. Each calcium atom is also coordinated by four H 2 0 molecules, the hydrogen atoms of which form the nearly cylindrical surface of the column. The channels contain four sites per formula unit with six calcium atoms, of which, in ettringite, three are occupied by S o d Z ions and one by two H 2 0 molecules. The repeat distance along the column is approximately 1.07 nm. Ettringite is trigonal, with a = 1.123 nm, c = 2.150 nm, Z = 2, D, = 1775 kg m - 3 (S59); the space group is P3 ic, apparent higher symmetry being due to twinning or disorder (M59). It is optically negative, with w = 1.463, E = 1.459 (M60). The doubling of c is due to ordering of the SO4'ions and H 2 0 molecules in the channels. Analogues with Fe3+, M n 3 + ,Cr3+ or Ti3+ in place of A13+ exist (B73,B74,S59); solid solution in the Fe3+A13+ series is almost continuous, but there is probably a small gap at 70-80 +
Crystal structure of ettringite. (A) Part of a single column in ( I 120) projection; A = Al, C = Ca, H = 0 of an OH group, W = 0 of an H,O molecule. Hydrogen atoms are omitted, as are the H,O molecules attached to those calcium atoms lying in the central vertical line of the figure. (B) Projection on the abplane, showing columns (large circles) and channels (small circles); the unit cell, with a = 1.123 nrn, is outlined. Modified from Struble (S59). Fig. 6.4
Hydrated Aluminate, Ferrite and Sulphate Phases
179
180
Cement Chemistry
lnole % ~ e " (B73). This Few phase has a = I . I182 nm, c = 2.2008 nm (S59). Analogues also exist with Cr0,'- replacing (B73.B75) and with S r 2 +replacing C a 2 + (B76). A carbonate analogue, c , A ~ , H , , . is well established (C28). Its unit cell ((I = 1.0834 nm, c = 2.1250 nm) (S59) and refractive indices (w = 1.480, E = 1.456) are near those of ettringite. but the cell parameters differ sufficiently to permit distinction by XRD. Hollow, tubular crystals have been observed (C28). Midgley and Rosaman (M57) concluded from DTA evidence that SO:can be partly replaced by O H - in the AFt phase of cement pastes. Pollman el a/. (P22a) showed that the fully OH--substituted phase, C,AH3,, exists but is easily carbonated. Material described as a silicate analogue was later shown to have also contained C 0 3 2 - (C28) but a product identified by Xray microanalysis as C,AS3H3, has since been found in reaction rims around C3A particles in pastes of that compound with C,S and gypsum (R20). The compound C,A.3CaC1,,30H20 has been reported to form at temperatures below O•‹C, but to be unstable at 20•‹C (S60,S61). Later attempts to prepare it failed (P22a). Several other AFt phases have been described (J18,P22a). A higher hydrate of ettringite, c,AS,H,,, is reported to exist at high relative humidities (P22a). Thaumasite, [Ca,Si(OH), i2H20](S04)(C0,) or c,sSCH, has a structure similar to those of the AFt phases, with Si4+ replacing A13 and SO4'and C 0 3 2 - groups in the channel sites (E2,E3). The octahedral coordination of the Si4+ was first established from IR evidence (M61). Thaumasite is hexagonal, with space group P63, a = 1.104 nm, c = 1.039 nm, Z = 2, D, = 1886 kg m - j (E2); the refractive indices are o = 1.470, E = 1.504. A Mn4+ analogue, jouravskite, exists (G55). Thaumasite and ettringite are not completely miscible, but limited solid solutions probably occur (EZ).
,, +
6.2.2
Properties
If crystals of natural ettringite are dehydrated, lattice shrinkage occurs, mainly in the ah plane, giving a product with a E 0.84 nm, c z 1.02 nm and probable constitution [Ca,AI(OH),~3H20],(S0,),; this has been observed using crystals of the natural mineral heated at 1 10•‹C(B77) and subsequently examined by XRD, or examined by electron diffraction, dehydration then occurring in the high vacuum (G56). Most of the molecular water has been lost, and the columns have fused together. Several phases of similar structure, such as despujolsite, [Ca3Mn(OH),.3H20](S0,), (G57), occur as natural minerals. Synthetic ettringite becomes almost amorphous when heated or subjected to a high vacuum, and gross morphological changes are
Hydrated Aluminate, Ferrite and Sulphate Phases
181
easily effected in the electron microscope. The differing behaviour may be related to defect concentrations. At ordinary humidities, ettringite begins to lose water rapidly at about 50•‹C. Fig. 6.2 includes a TG curve, determined at 10 deg C min-'. Curves obtained at lower heating rates for ettringite and its Fe" and Cr3+ analogues have been reported (B73). Thaumasite is thermally more stable, rapid weight loss beginning only at 110•‹C(B78). DTA curves have been reported for ettringite (M57,B73), its Fe3+ (M57,B73) and C03'- (C28) analogues and thaumasite (B78). Fig. 6.3 includes a curve for ettringite. All show strong endotherms at 110-1 50•‹C, the temperature varying somewhat with the composition, technique and amount present. When present in cement pastes, ettringite typically gives a peak at 125-130•‹C (B70). In XRD powder patterns, ettringite is readily recognized by its strong, low angle peaks at 0.973 nm (10i0) and 0.561 nm (1 120), which disappear on heating or intensive drying at room temperature. The corresponding peaks of thaumasite have spacings of 0.956nm and 0.552 nm. Some investigators have found it impracticable to determine ettringite quantitatively by QXDA, because crystallinity is easily lost on grinding (M62,015), but Crammond (C30) reported satisfactory techniques for determining ettringite, thaumasite and gypsum by this method. Ettringite has also been determined by DTA or DSC (015). 1R spectra of ettringite, thaumasite and related phases have been reported (B73-B75,M61 ,B78,S59,B70). 6.3 Other hydrated phases
6.3.1 Hydrogarnet phases These phases have structures related to that of grossular or garnet (Ca3AI,Si3012).The latter has a cubic structure, in which the silicon, aluminium and calcium atoms are in tetrahedral, octahedral and distorted cubic coordination, respectively; each atom is bonded to one silicon, one aluminium and two calcium atoms. In the hydrogarnets, this structure is modified by omission of some or all of the silicon atoms, the charge being balanced by replacing each of the oxygen atoms to which it was attached with a hydroxyl group. The A13+ may be partly or wholly replaced by Fe3+. Complete replacement of silicon by four hydrogen atoms in grossular thus gives Ca,[AI(OH),], or C3AH,, and solid solutions exist within a compositional region bounded by C,AH,, C3FH6,C3AS, and C3FS3. In recent mineralogical nomenclature, phases in the C3AH6-C3AS3 series are collectively called hydrogrossular, and the names katoite and hibschite are used more specifically to denote those in the C3AH6-C3ASl,,H3 and C3ASl.,H3C3AS3 ranges, respectively (P23).
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Cement Chemistry
C,AH, is the only stable ternary phase in the Ca0-AI20, H,O system at ordinary temperatures, but neither it nor any other hydrogarnet phase is formed as a major hydration product of typical, modern Portland cements under those conditions. Minor quantities are formed from some composite cements and, in a poorly crystalline state, from Portland cements. Larger quantities were given by some older Portland cements, and are also among the normal hydration products of autoclaved cement-based materials. C,AH, is formed in the 'conversion' reaction of hydrated calciunl aluminate cements (Section 10.I). Hydrogarnet phases crystallize in various cubic forms. of which at ordinary temperatures icositetrahedra are probably the most usual. The space group is Ia3d, with Z = 8; C3AH, has (1 = 1.25755nm. D, = 2527 kg m-3 (K28). Detailed structural studies have been reported on C3AH, or C,AD, (K29,F27,B79) and on phases in the C3AH,-C,AS, (B80.S62) and C,FH,C,FS3 (C31) series. The hydrogen atoms lie outside the tetrahedra of oxygen atoms to which they are attached, and which surround the empty silicon sites. They form neither hydrogen bonds nor, contrary to some early views, H, clusters. The compositions of naturally occurring specimens indicate that the C3AH6-C,AS3 solid solution series is continuous (P23). but synthetic studies show that it is unlikely that all compositions in this series can be obtained at any one temperature; probably, those in the range from C3AH, to about C,AS2H, can be obtained at relatively low temperatures, and the more siliceous ones only above about 360•‹C(R6). Contrary to many earlier opinions, C3FH6 can be prepared in the absence of StO,, though it is metastable and easily decomposed at ordinary temperatures (R21). Equations relating the cell parameter and coordinates of the oxygen atoms in hydrogarnets to composition have been proposed (B80,B81), but give mediocre agreement with the observed cell parameter for C,AH,. The equations implicitly assume that Vegard's law is obeyed over the entire C3AH,-C3AS, series, but there is probably a break near the C3AS, end (R6). An empirical equation giving better agreement over the more highly hydrated compositions relevant to cement chemistry (Si02/Ca0 Q 0.67) is
where a is the cell parameter in nanometres and Fe,03 and H,O are the values of x and y, respectively, in the formula C,A, -,F,S,-, ,H,. In order : to determine the composition of a phase of this general composition from : the XRD powder pattern, a second quantity must be used. Zur Strassen (Z10) showed that the intensity ratio of the 022 peak to the 116 peak (d = : 0.4445 nm and 0.2040 nm, respectively, for C,AH,) was suitable. Table 6.4 1
1
j
184
Cement Chemistry
or 5.5 on drying over P,O,. T G curves at 0.5 or 5 deg min- ' (L25) show no definite steps; at 1 1O0C, H/A is about 4 for the slower, or 6 for the faster heating rate. On static heating at p ( H 2 0 ) = 6 torr, H/A is about 6 at 50JC, 3 at 1 10•‹Cand I at 200•‹C. The products of thermal decomposition depend on how easily the water can escape (B83). If escape is easy, a nearly amorphous product retaining some of the structural features of CAH,, is formed, but if it is not. C,AH, and gibbsite (AH,) are formed. The latter situation can occur if a hardened paste of a calcium aluminate cement containing CAH,, is heated. Under conditions not giving C,AH6, the X R D pattern varies continuously with temperature; the crystallinity deteriorates, the cell parameters decrease slightly and relative intensities change markedly. Little change occurs up to about 70•‹C, but by 120•‹C(H/A 1. 2.5), only the 1.4 nm peak and a close doublet near 0.7 nm remain, and the relative intensity of the 1.4 nm peak is much reduced. By 200-350•‹C the material is amorphous; by 1000LC,CA is formed. The marked loss in crystallinity that occurs when H/A falls below 4 can be attributed to the fact that further loss entails dehydroxylation. With some preparations small amounts of C2AH, and AH, are also formed. The rehydration behaviour has been studied (B84). DTA curves at 10 deg C min - show a large endotherm at 130- 150•‹Cand a smaller one at about 290•‹C. Since the first of these is due to loss of molecular water, its height is affected by any preliminary drying that the specimen may have undergone. DTA has been used to determine the relative amounts of CAH,, and AH,, and thus indirectly also of C,AH6 in calcium aluminate cement concretes, but caution is needed because the CAH,, may have undergone partial dehydration and also because its thermal decomposition can itself yield AH,. T o some extent, this caution also applies to the determination of CAH,, by QXDA. A 27Al N M R study (G54) confirmed the octahedral coordination of the aluminium atom. Henning (H34) reported IR spectra for CAH,,. CAH, and CAH,.
6.3.3 Brucite, hydrotalcite and related phases Brucite [magnesium hydroxide; Mg(OH),] is isostructural with CH. It is formed in Portland cement concrete that has been attacked by magnesium salts, and on hydration of Portland cements high in MgO and possibly of Portland cements in general. It has a = 0.3147 nm, c = 0.4769 nm, Z = 1, D, = 2368 kg m-,, o = 1.561, E = 1.581 (S63). Three polytypes of a h minium hydroxide [Al(OH),], gibbsite, bayerite and nordstrandite. contain layers essentially similar to those in brucite, but with an ordered pattern of
(
;2
Hydrated Aluminate, Ferrite and Sulphate Phases
185
cation vacancies and a different relationship between adjacent layers, resulting from hydrogen bonding. There exists a range of phases structurally related to brucite as the AFm phases are to CH; that is, some of the Mg2+ ions are replaced by tripositive ions, typically A13' or Fe3+, and the charge is balanced by anions which. together with H,O molecules, occupy interlayer sites. Allman (Al I ) reviewed them. The first structure determination on this group of phases was (A 12). on sjogrenite, a polytype of [Mgo.,5Feo,25(OH),1(C03)o.,25(H,0)o.s They are sometimes called hydrotalcite-type phases, after a polytype of the A13+ analogue of sjogrenite. They are formed on hydration of slag cements, and as minor hydration products of Portland cement. These products possibly approximate in composition to meixnerite, [Mg,,,,AI, (OH)2](OH),,2,(H,0)o,5 (K30), which, like the other phases named above. occurs as a natural mineral. Because Mg2+,A13+ and Fe3+ are all relatively similar in radius, the ratio of di- to tripositive ions is not fixed, as in the AFm phases. Brindley and Kikkawa (B85) discussed the factors limiting it. The minimum M 2 + / (M2+ M 3 + )ratio is 213, and could be set by a requirement to avoid the occurrence of M3' ions in adjacent sites; in phases of this ratio, the cations are ordered (H35). giving unit cells with a approximately J3 times that of brucite. At higher ratios, the cations are disordered, though ordered regions may occur locally, and the upper limit of the ratio may be set by nucleation of pure brucite layers if the proportion of tripositive ions falls below a certain value. In a study of synthetic meixnerites, pure phases were obtained only at Mg/(Mg Al) ratios between 0.67 and 0.76, suggesting an upper limit close to 0.75 (M64), and it is probably significant that this ratio is much the most usual in the natural minerals. As with the AFm phases, anion exchange reactions occur (H35), and meixnerite readily takes up CO,, giving a material similar to hydrotalcite. Among the natural minerals of the group, ones with interlayer C0,'- are relatively common, and ones with interlayer O H - very rare. This may be a further indication that replacement of O H - by C 0 , 2 - occurs easily. Differing schemes for the packing of H,O molecules and C03'- ions in the interlayer have been proposed (A 12,18). Layer thicknesses are similar to those of the AFm phases, and it would be difficult to distinguish the two groups of phases by XRD if only the basal reflections could be observed. Polytypism is common; for example, sjogrenite, with a two-layer structure, has a three-layer polytype, called pyroaurite. Three-layer structures are the ones normally formed at room temperature. Meixnerite, which is a three-layer form, has a = 0.30463 nm, c = 2.293 (3 x 0.764) nm, space group ~ 3 m Z, = 3, D, = 1950 kg m - 3 (K30). Thermal dehydration (B86) and IR spectroscopic (H35) studies on Mg-A1 phases of this group have been reported.
,,-
+
+
186
6.3.4
Cement Chemistrv
Sulphate phases
Table 6.5 lists these phases and gives crystal and optical data. The gypsum used in cement manufacture is usually of natural origin. but byproduct gypsums produced by the chemical industry have also been used. The most important is phosphogypsum, resulting from manufacture of phosphoric acid by the wet process. It contains impurities, especially phosphate and fluoride, that retard setting and which, if present in relatively high proportions, may have to be partially removed by appropriate pretreatments (M65). Another byproduct gypsum, which is likely to be produced in increasing quantity, is that resulting from flue gas desulphurization. Dehydration of gypsum in air at 70-200•‹C gives hemihydrate (CaSO,. 2. 0.5H20)or y-CaSO, ('soluble anhydrite'), the extent of dehydration depending on the temperature and duration of heating and RH of the surrounding atmosphere. y-CaSO, is probably never completely anhydrous (B88). These products have essentially identical crystal structures, which are hexagonal or pseudohexagonal, with the H 2 0 molecules in channels parallel to the hexagonal c-axis. The channels can accommodate methanol (R23). Hemihydrate is known as a natural mineral, called bassanite. Above about 200•‹C,anhydrite (also called 'insoluble anhydrite'; Section 2.5. I ) is formed. Reported a and P forms of hemihydrate appear to differ only in degree of crystallinity or crystal size, the more crystalline a form being obtained by dehydration of gypsum in the presence of liquid water or aqueous solutions and the form by heating in air (B89). y-CaSO, is readily rehydrated to hemihydrate. There has been uncertainty as to the maximum H,O content of these products and whether variation in this quantity is continuous. Kuzel (K31), studying highly crystalline preparations, found that variation was continuous from 0.53 to 0.62 H,O, but that a miscibility gap existed at 0.030.53 H,O; the space group was P3,21 for 0.62 H 2 0 and 12 for 0.53 H,O. Some studies indicate a maximum H,O content well above 0.5; thus Abriel (A13) reported a crystal structure refinement for a preparation with 0.8 H,O, for which the space group was P3,21. The H 2 0 molecules were found to be statistically distributed, with four molecules among five positions. Abriel concluded that larger unit cells arose through ordering of the H 2 0 molecules, which was possible only at H,O contents of or below 0.5. This would account for the larger cell with space group I2 found at H,O contents around 0.5 and for the reversion to a smaller cell in y-CaSO,. The extent to which these structural distinctions are relevant to the less crystalline products obtained by heating gypsum in air is uncertain. Reaction in the presence of water between K2S04 and CaSO, can produce syngenite, KCS,H, data for which are included in Table 6.5.
188
Cement Chemistry
6.4
Equilibria and preparative methods
6.4.1
The CaS0,-H,O, systems
CaS0,-Ca(OH),-H20
and CaS04-K2S04-H20
At ordinary temperatures, the metastable solubility of hemihydrate or y-
CaSO, is considerably higher than the solubility o f gypsum, which is thus precipitated on mixing either of the former materials with water (Fig. 6.5); the setting of plaster is based on this reaction. Anhydrite displaces gypsum a s the stable phase above 42"C, o r at somewhat lower temperatures if other solutes are present, but is not readily precipitated, and gypsum can persist metastably up to at least 98"C, above which temperature hemihydrate is stable relative to gypsum.
Temperature Fig. 6.5 The CaS0,-H,O
(OC)
system. Arter Posnjak (P25).
The solubility of gypsum is only slightly decreased in the presence of CH, and vice versa (Table 6.6). The effect, which could scarcely be explained if the only dissolved species were C a 2 + , O H - and SO,'-, occurs because substantial proportions of the O H - and SO,'- ions are complexed to give C a O H t (Section 5.5.2) and neutral CaSO,, respectively. The formation constant of the latter species is 204 mol I - ' (S51). Gartner et ul. ((358) discussed equilibria governing precipitation of gypsum, CH and syngenite with reference t o the early stage of cement hydration. They concluded that the equilibrium activity products at 25OC mo13 1 - 3 mo12 1 -'for gypsum, 8.25 x were (2.547 2.2581) x
+
Hydrated Aluminate, Ferrite and Sulphate Phases
189
for CH and (13.91 - 0.3) x l o 5 mo15 1 - 5 for syngenite. where I is the ionic strength. As Bailey and Hampson (B90) noted earlier, it is essential to take account of both complex formation and activity coefficients. Table 6.6
Invariant points in the systcm CaO-AI,O,-SO,-H,O U91). Labelling of points as in Fig. 6.8
at 20'C (D24.
Concentrations (mniol 1 Point
CaO
Solid phases Ca(OH), Hydrous alumina Gypsum Ca(OH)2,C,AH,, C,AH,,. C2AH, C,AH,. hydrous alumina CH. gypsum Hydrous alumina, gypsum Ettringitc, gypsum, Ca(OH), Ettringite. gypsum, hydrous alumina Ettringite. monosulphate, Ca(OH), Ettringitc. monosulphatc, hydrous alumina
6.4.2 The CaO-AI,03-H20, CaO-A120,-SO3-H,O
AI,O,
CaSO,
20.7 -
21.5 10.9 5.79 19.7 -
20.6 15.2 21.1 5.98
Ca0-A1203-Si0,-H20
and
systems
Fig. 6.6 shows equilibria in the CaO-A120,-H20 system mainly as found by Jones and Roberts (517) at 25•‹C and reported by Jones (J l9), who also reviewed earlier studies. Table 6.6 gives invariant concentrations at 20•‹C. The stable phases in contact with solution at 25•‹C are gibbsite (AH,), C,AH, and CH. but metastable solubility curves for AFm phases, CAH,, and hydrous alumina can also be obtained. The curve shown for AFm phases contains a discontinuity at Y. Jones and Roberts considered that the curve CY related to C,AH,,, and the curve YT to a solid solution of C,AH,, with C,AH, which had CIA = 2.0 at T and C/A = 2.4 at Y; the prolongation TT' of this latter curve related to C2AH,. The results of Dosch and Keller (D20) do not support the conclusion that solid solutions with CIA ratios of 2.0-2.4 exist, but the data of Jones and Roberts could probably be equally well explained by the formation of oriented intergrowths on a submicrometre scale, which in the present case would be virtually indistinguishable from solid solutions by XRD, light microscopy or solubility relations. Not all workers have observed the discontinuity at Y. The curve for the AFm phases may be slightly variable,
190
Cement Chemistry
as small changes in conditions could determine whether separate crystals of C,AH,, and C2AH,, intergrowths or solid solutions are formed. Data have also been obtained at other temperatures from I C to 50'C (519); some are considered i n Section 10.1.5. At 50•‹C or above. the other ternary phases are rapidly replaced by C,AH,.
CaO C O N C E N T R A T I O N ( m m o l I-') Fig. 6.6 The CaO-AI,O,-H,O system at 25•‹C. mainly aftcr Joncs and Roberts (J17); CAH,, solubility curve at 2 1 • ‹ C Percival and Taylor (P26).
No detailed studies on the Ca0-A120,-Si0,-H20 system at ordmary temperatures have been reported, but, based largely on considerations of solubility products, Dron (D23) suggested the main features of a probable metastable equilibrium diagram, a modified form of which is shown in Fig. 6.7. The diagram indicates the metastable coexistence of CH, C-S-H and C,AH,,, and that stratlingite cannot coexist with CH. In cement pastes, the AFm and AFt phases appear to accommodate some silicon; the extents of these substitutions, and the related question of the scale on which the hydrated phases are mixed, are considered in Sections 7.2 and 7.3. Equilibria in the CaO-A120,-SO,-H,O system were studied at 25'C by Jones jJ20) and at 20•‹C by d'Ans and Eick (D24). Following Brown (B91), the tatters' results are shown on a three-dimensional diagram in which the
Hydrated Aluminate, Ferrite and Sulphate Phases
191
functions plotted are the tenth roots of the concentrations (Fig. 6.8). This tigure represents the metastable equilibria involving AFm phases and hydrous alumina. The stable equilibria, which involve C,AH, and crystalline AH,, were also studied but are less relevant to cement hydration chemistry. Table 6.6 gives data for the invariant points and probable AFm compositons. Seligmann and Greening (S56) considered that an AFm solid solution may have a stability field in the quaternary system. The CaOAI,O,-SO,-H,O system modified by various additions of alkalis has also been studied (521). In the main, the general form of the phase relations appears to be unchanged, but the invariant concentrations are considerably altered.
Fig. 6.7 Suggested general form of the metastable equilibria in the CaO-AI,O,-
SO,-H,O system at ordinary temperature, showing solubility surfaces. Modified from Dron (D23).
Ogawa and Roy (016) studied the stability of ettringite in the presence of water at elevated temperatures and summarized earlier work. Ettringite appears to be stable in water up to a t least 90•‹C a t atmospheric pressures, 130•‹Cat 6.7 MPa or 145•‹Cat 27 MPa. The principal decomposition product is monosulphate.
Cement Chemistry
( CaSO4
C
I
K CaO
Fig. 6.8 Metastable equilibria in the CaO-AI,O,-SO,-H20 system at 20 C. showing crystallization surfaces, based largely on the data of d'Ans and Eick (D24). Concentrations are expressed as logarithms to the base 10 of values in mol I - ' and,
following Brown (B91). are plotted on scales proportional to thcir lcnth roots. Concentrations at the lettered invariant points are given in Table 6.6. 6.4.3
Preparative methods
Mylius (M60) described the preparation of many calcium aluminate hydrates and related compounds. As with C-S-H, it is normally essential to exclude atmospheric CO, and to avoid prolonged contact with glass apparatus. Dosch and Keller (D20) described methods for obtaining many AFm phases by anion exchange or other special procedures. The C,A and C,A hydrates are most conveniently prepared by adding C a O o r saturated C H solution to a supersaturated calcium aluminate solution obtained by shaking CA or white calcium aluminate cement (Section 10.1.1) with water. Such solutions typically contain up to about 1.2 g CaO 1- and 1.9 g AI,O, I-', these concentrations depending on the shaking time, temperature, proportioning and particle size of the starting
'
Hydrated Aluminate, Ferrite and Sulphate Phases
193
material. The amount of CaO or CH solution added must be such as to produce the desired compound and a solution in metastable equ~librium with it, in accordance with the solubility relations (Fig. 6.6 and Table 6.6). CAH,, may be prepared in this way at 5 • ‹ Cno added CaO or CH being added. C3AH, may be prepared from metallic aluminium and CH solution at 50•‹C. or by hydrothermal treatment of C3A or a mixture of CaO and AH, with water at 150•‹C. Ettringite may be prepared by mixing solutions of CH, gypsum and A12(S04),. Mylius used 1 I of saturated CH (1.2 g CaO 1 - I ) diluted to 1.2 1 (A), 530ml of saturated gypsum (2.05gCaS04 I-'; B) and a solution of 1.78 g of AI,(SO,), 18 H 2 0 in 770 ml of water (C). Solutions A and B were first mixed, and solution C then added, with shaking. The mixed solution was shaken at 18•‹Cuntil an initially precipitated gel had been replaced by crystalline material. The molar proportions are 8 C a 0 : IAI,(SO,), : 3CaS04. Monosulphate is troublesome to prepare. Mylius mixed 5OOml of a supersaturated calcium aluminate solution (649 mg C a 0 and 969 mg Al,O, I-') at 18"C, with shaking, with 2924m1 of saturated CH (1.25 g CaO I - ' ) and 334 ml of saturated gypsum (1.93 g CaSO, I - '). After 30 min, crystallization was complete; the mixture was filtered, washed four times with a little water followed by 96% ethanol and ether, and dried over CaCI, and soda lime without evacuation. Monosulphate may also be prepared hydrothermally (A8). Carlson and Berman (C28) described the preparation of C,ACH,, and c,AC,H,,. c , A ~ H , , was obtained using a supersaturated calcium aluminate solution, saturated CH and Na,CO,. The most satisfactory method for preparing c,A~,H,, employed a supersaturated calcium aluminate solution, NH,HCO, and a concentrated CH solution containing sucrose. Crammond (C32) reviewed reports of the formation of thaumasite in the laboratory and in deteriorated building materials. It forms readily at 4 C from mixtures that contain the appropriate ions and also 0.&1.0% of reactive alumina. It does not form at 25•‹Cor above. A little AI3+ may be essential, and reported failures to reproduce published syntheses may be due to the use of aluminium-free materials.
6.5 6.5.1
Hydration reactions of the aluminate and ferrite phases
Reaction of C,A with water or with water and calcium hydroxide
The reactions of C,A and C,AF with water, alone or in the presence of calcium hydroxide or sulphate or both, have been widely studied for the light they may cast on the mechanism of Portland cement hydration, and
194
Cement Chemistry
especially on the action of gypsum in controlling setting. The results of such studies must be interpreted with caution. The aluminate and ferrite phases in Portland cement clinker difrer greatly in composition from the pure compounds. In normal clinkers, they are intimately mixed with each other and react in an environment chemically and microstructurally influenced. and perhaps dominated. by the silicate phases, and which is further afTected by the presence of alkalis. The hydration of CA and C I 2 A , has similarly been studied for its bearing on that of calcium aluminate cements, and is considered in Section 10.1.4. Many studies have shown that the reaction of C,A with water in suspensions at ordinary temperatures gives C2AH, and C,AH which are converted into C,AH, at a rate that depends on such factors as the temperature, w/s ratio, grain size, and presence or absence of CO?. At temperatures above about 30•‹C or in pastes, conversion is rapid and the AFm phases may not be observed; local temperature increase due to the strongly exothermic nature of the reaction probably has an important effect. The formation of gel as a precursor to the AFm phases, mentioned in several early reports, has been confirmed by SEM and XRD (B92) and HVTEM (S41) studies, which indicate that the initial product consists of thin foils, apparently growing on the C,A surfaces, and not detectable by XRD. Their composition is not known. ESCA evidence (522) suggested that the initial product has a relatively high CaO/A120, ratio, possibly equal to 4. On the other hand, the CaO/A1203 ratio of the solution rises above 3.0 when precipitation begins, indicating that a solid product with CaO/AI20, below 3.0 has been formed (B93,G59). Another ESCA st.udy (B94) indicated that the surface layer of the starting material is hydroxylated and deficient in calcium before coming into contact with liquid water. Inspection of Fig. 6.6 indicates that the first product to form might be C,AH, or some form of hydrous alumina, the latter tending to redissolve later. In either case. this would explain the observed increase in the CaO/A1203ratio of the solution. In pastes, the hydration of C,A, as followed calorimetrically. is slightly retarded if CH is present (C33). The products are essentially the same as those formed in the absence of CH, but the crystals of AFni phases are smaller (G60). In dilute suspensions, the first crystalline product to form is C,AH,,; this causes the CaO/A120, ratio in the solution to decrease (B93).
,.
6.5.2 Reaction of C,A with water in the presence of calcium sulphate
Studies using calorimetry, electron microscopy, XRD and DTG (S64.S65, C33) have shown that this reaction occurs in two stages, both of which are strongly exothermic. The first stage is characterized by a peak in the heat
Hydrated Aluminate, Ferrite and Sulphate Phases
195
evolution curve during the first 30 min, and yields ettringite. In the second stage, which is characterized by a peak typically occurring at 2 4 4 8 h, the cttringite reacts further and AFm phases are formed. The reactions may be represented by the following equations:
where the C,AH,, could occur either in a solid solution with monosulphate or in separate crystals. In ordinary Portland cements, the ratio of SO, to AI20, is typically about 0.6, suggesting that the final products will be C,ASH and C,AH, but some AFt phase nevertheless often persists. Studies by TEM (B95) and HVTEM (S41) show that, as in the absence of gypsum, the formation of a distinctly crystalline reaction product is preceded by that of a layer of amorphous or poorly crystalline material having a foil or irregular, platey morphology on the surface of the grains. There is no direct evidence concerning the composition of this material, but Brown (B91) concluded from a consideration of the phase equilibria (Fig. 6.8) that. depending on the concentrations in the solution in contact with the C,A when the latter began to react, the first A120,-containing phase to forrn might be either AH, or ettringite. The morphology of the product does not support the view that it is ettringite, though it might be amorphous material of similar composition. The ettringite produced in the first stage of reaction tends to form as stubby, prismatic crystals up to 1 pm long and close to the C,A surfaces, but it is also formed away from these surfaces, thus indicating a through solution mechanism (M66,S41). Further evidence for the latter is provided by the obsertztion that, in pastes of C,A and C,S with gypsum, some of the C,A grains form hollow shells only partly occupied by unreacted starting material or hydration products (S41). The ettringite crystals are readily damaged by the electron beam. Longer needles, of high aspect ratio, can also form. The amount of available space (M66), pH (B95) and sulphate ion concentration (S41) have been variously considered to control the morphology. The calorimetric evidence, and other evidence discussed in Section 7.6.2. indicates that reaction of the C,A is markedly retarded in the presence of gypsum and that gypsum and CH together are more effective than gypsum alone (C33). The effect of gypsum has been attributed to the protective action of a layer of ettringite (S65,C33), but it has been queried whether this layer is sufficiently impermeable to have such an effect (M66). Retardation
,,
,,
196
Cement Chemistry
has also been attributed to an underlying layer of hydrous alumina (C34) or AFm phase (P27,G61); this view is consistent with the electron microscopic evidence mentioned earlier. The retarding properties of such a layer might well depend on its composition, thus explaining the different rates of reaction observed in the absence of admixtures and with CH or gypsum or both. Retardation has also been attributed to the blocking of active sites on a modified C3A surface by adsorbed sulphate ions (S66,F28); however, Na2S04 has no significant effect on the rate of reaction (C33). The reaction of the aluminate phase in a cement paste is influenced by the presence of the silicate phases and of alkali (Section 7.6). Ghorab and El Fetouh (G62) studied the effect of the latter on reactions in the pure system.
6.5.3
Reaction of the ferrite phase
Many investigations have shown that the hydration products of phases in the C,(A,F) series are essentially similar to those formed from C3A under comparable conditions (C35,S57,C36,DIO,C37). The experimental evidence includes XRD, DTA, DTG, TEM, SEM, IR and calorimetry. The first crystalline products to form in the absence and presence of CaSO, are AFm and AFt phases, respectively, the AFt phases being later replaced by AFm phases, as in the case of C3A. Both types of product phase contain Fe3 as well as At3+, and tend to undergo further change to give hydrogarnet phases. ESCA evidence (B94) indicates that the surface of C,AF is hydroxylated and deficient in calcium and iron before contact with liquid water. The deficiency in iron possibly arises from zoning, considered in Section 2.3.1. Different preparations of ferrite phase of a given composition and particle size distribution appear to vary greatly in reactivity (e.g. Ref. S41), perhaps because of zoning and differences in distribution of cations among tetrahedral and octahedral sites (Section 1.5.1). Some of the earlier studies indicated that reaction is accelerated in the presence of CH, but later work showed that, as with C3A, reaction is retarded by CH, more strongly by gypsum and still more strongly by the two together. Under comparable conditions, C4AF appears normally to react more slowly than C3A, though initially at least, the reverse may sometimes be the case (S41). It appears generally agreed that the rate of reaction of the ferrite phase decreases with the Fe/AI ratio. Several investigators have noted that the AI/Fe ratio in the product phases tends to be higher than that of the starting material, and have concluded that an iron(II1) oxide or hydroxide is also formed. Teoreanu et al. (T33), using Mossbauer spectroscopy and XRD, concluded that the hydration products formed from C 2 F or C4AF at ordinary temperatures included FH,. With C2F above 75"C, this was replaced by hematite. Fukuhara et al. (F29) +
Hydrated Aluminate, Ferrite and Sulphate Phases
197
concluded from calorimetric evidence that the AFt phase formed from C4AF in pastes at 20•‹Chad an AI/Fe ratio of about 3 and postulated that FH, was also formed. Rogers and Aldridge (R21) found that in C2(A,F) preparations hydrated at 4•‹C the only product detectable by XRD was C4(A,F)H,,, but SEM with X-ray microanalysis evidence showed that an amorphous iron oxide or hydroxide was also formed. In products formed at 8O0C, hematite was identified by SEM with X-ray microanalysis, XRD and Mossbauer spectroscopy. Hydrogarnet phases formed from C,A,F or C4AF contained only small proportions of iron as did that formed from C6AF, at 150•‹C;those formed from C6AF, at 21•‹Cor 80•‹Chad Fe/(AI + Fe) ratios of 0.30-0.35. Fortune and Coey (F30), using XRD and Mossbauer spectroscopy, similarly found that hydrous iron(Il1) hydroxide was present among the products of C4AF hydrated at 72•‹C. The hydrogarnet phase that was the main product had an Fe/(AI Fe) ratio of 0.22. If CH was added, this ratio was increased to 0.32, but the hydrogarnet formed in presence of gypsum was almost free from Fe3+. Brown (B96) concluded that in C4AF hydration in the presence of CaSO,, the early product was C2AH, or hydrous alumina, and that it was followed by formation of an AFt phase containing little or no Fe3+;an iron oxide gel containing some Ca2+ was also formed. The conclusion regarding the later products agrees substantially with that of Fukuhara et 01. (F29). The tendency to form iron oxide or hydroxide may be connected with the fact that species containing A13+ can migrate in pastes relatively easily, whereas those containing Fe3 cannot, and are thus largely confined to the space originally occupied by the anhydrous material from which they come (T19).
+
+
6.5.4
Enthalpy changes
Table 6.7 gives standard enthalpies of formation of some relevant compounds, and Table 6.8 gives enthalpy changes for hydration reactions of C,A calculated from them. The strongly exothermic nature of these reactions is apparent. Lack of data for the standard enthalpies of the hydration products containing iron precludes similar calculations for the ferrite phase. For the hydration of C,A to give C3AH6, Lerch and Bogue (L24) obtained an experimental value of -234 kJmol-'; other workers have obtained similar results. Experimental determinations of the enthalpy of hydration of C4AF have given -203 kJ mol- (pr-oducts not established; Ref. L24), and, in the presence of excess CH to give hydrogarnets, - 193 kJ mol-' for C,AF and -208 kJ mol-' for C4Ao,6,Fo,32(B60). For C4AF in the presence of gypsum to give ettringite and FH,, a value of - 352 kJ mol-' has been
'
198
Cement Chemistry
reported (F29). Houtepen and Stein (H36) gave standard enthalpies of formation of some other AFm phases determined experimentally, and Babushkin E I (11. (B62) gave values for other compounds estimated on the basis of crystal chemistry. Standard enthalpies of formation (AH',; kJ mol-') for some compounds relevant to hydration of the aluminate and ferrite phases (25 C except where otherwise stated)
Table 6.7
Compound
AH;
Ref.
Compound
A HI,:
Ref.
ChA2F CSH, CSH, (P) CH
-8104.9 -2022.6 - 1574.65 - 986.1
NII W 10 W 10 WIO
C4AH 1 3
-8318 - 8 176 - 5548 -285.83
H36 WI0 wI0 W10
,
C4ACH
10 6 8
C3AHh
H 2 0 (liqu~d)
" Using data for oxides. from Ref. WIO.
Enthalpy changes for some hydration reactions of C,A. calculated from the data in Table 6.7
Table 6.8
Reaction
AH (kJ per mole of C3A)
Hydration of Portland cement 7.1 G e n e r a l d e s c r i p t i o n o f t h e h y d r a t i o n p r o c e s s a n d p r o d u c t s 7.1 . I
Evidence from X-ray diffraction
Unless otherwise stated, this chapter relates to ordinary Portland cements hydrated in pastes at 15-25•‹C and w/c ratios of 0.45-0.65. X R D powder studies on such pastes have been reported by many investigators (e.g. C38,M67). The rates of disappearance of the phases present in the unreacted cement are considered more fully in Section 7.2.1. Gypsun1 and other calcium sulphate phases are no longer detectable after, at most, 24 h. and the clinker phases are consumed at differing rates, alite and aluminate phase reacting more quickly than belite and ferrite. The ratio of belite to alite thus increases steadily, and after about 90 days at most, little or no alite or aluminate phase is normally detectable. The peaks of CH begin to appear within a few hours. Within a few days they dominate the X R D pattern. A diffuse band at 0.27-0.31 nm and a sharper one at 0.182 nm develop more slowly; these are similar to the bands of C-S-H observed in C,S or p-C,S pastes (Fig. 5.6). but taking into account the amount of C-S-H likely to be present, they suggest an even lower degree of crystallinity than in the latter materials. The hydrated aluminate phases that are observed depend on the cement composition and on the time and other conditions of hydration. With most ordinary Portland cements under the conditions mentioned above. ettringite peaks are detectable within a few hours and increase in intensity to a maximum at about 1 day. They then tend to weaken, and may disappear completely, though with many cements they persist indefinitely. The 0.973and 0.561-nm peaks are especially prominent, but many other peaks can often be detected, indicating that at least some of the ettringite is highly crystalline. Peaks of AFm phases typically begin to appear after about 1 day and become stronger as those of ettringite weaken. This behaviour is similar to that observed with pastes of C,A (Section 6.5.2) and can be explained in the same way. The only peaks clearly attributable to AFm phases are usually those at about 0.288 nm (1 1.0), 0.166 nm (30.0) and the first- and second-order basal reflections corresponding to one or, usually, more layer thicknesses. Taking
200
Cement Chemistry
into account the amounts likely to be present, the X R D evidence suggests that much of the AFm material is poorly crystalline. If the specimen is kept reasonably moist (RH 2 80%) and no special precautions are taken to exclude CO, during specimen preparation and examination, the principal layer thickness is usually 0.89 nm, attributable to c,ASH,, or its solid solution with C4AH13,and weaker peaks corresponding to layer thicknesses of approximately 0.82 nm, 0.79 nm, 0.76 nm, or any combination of these. The 0.82-nm spacing could be due to c,AC,,,H,, or c,ASH,, or both, the 0.79-nm spacing most probably to C4AH13 and perhaps also to hydrotalcite-type material, and the 0.76nm spacing to c,ACH,,. The phases containing C 0 , 2 - form by carbonation of C,AH,,, and the c,ASH,, by partial dehydration of c,ASH,,. In all these formulae, the possibility of some replacement of A13+ by Fe3 is assumed. To establish the nature of the AFm phases in the untreated paste with more certainty, it will often be necessary to prepare and examine the specimen under C0,-free conditions and at more than one relative humidity. +
145 165 UNHYDRATED 4 8 5
Po4 15 M I N
1 HOUR
480
530
Fig. 7.1 (B97).
DTA curves for pastes of a typical Portland cement. Bensted and Varma
Copeland et al. (C38) observed that the XRD patterns of many pastes included peaks of a hydrogarnet in addition to those of other hydrated aluminate phases, and obtained evidence that this had formed by hydration.
Hydration of Portland Cement
201
of the ferrite. The unit cell parameter was 1.24 nm. Hydrogarnet phases do not seem to form in any quantity at ordinary temperatures from modern cements. Luke and Glasser (L27) found a hydrotalcite-type phase by XRD in a residue from which the other phases had been removed by chemical extraction, and both a hydrotalcite-type phase and a poorly crystalline product probably approximating to a hydrogarnet have been found by TEM (Section 7.1.3). Both appear to be minor products. Calcite, if present, is detectable by a sharp peak superimposed on the maximum in the C-S-H band at 0.3035 nm, and if in sufficient quantity by additional peaks.
7.1.2
Evidence from differential thermal analysis and infrared spectroscopy
Bensted and Varma (B97) summarized the use of these methods for the examination of cement pastes. Both are mainly useful for following the phase changes in relatively young pastes. Fig. 7.1 shows DTA curves for pastes of a typical Portland cement at ages up to 7 days. The peaks at 145•‹C and 165•‹Cgiven by the unhydrated cement are due to gypsum, and that at 485•‹C to a small amount of CH formed during storage, probably by hydration of free lime. The peak at 135-140•‹C that appears within 15 min is due to ettringite; its intensity passes through a maximum at 1 6 2 4 h. The peak at 1 1 5-125•‹C is due to C-S-H or cement gel, and that at 530-550•‹C to CH. The relatively large crystals of this phase that are formed on hydration in the paste decompose at a higher temperature than the smaller ones formed on hydration during storage. The peak at 185-200•‹C is due to AFm phases. One would expect all these peak temperatures to vary with the technique used, but that relative positions would be essentially unchanged. In the first effective studies of hydrated cements by DTA, Kalousek and co-workers (K27,K32,K33) observed the early formation of ettringite and its subsequent replacement by what was termed a solid solution, but which was probably a mixture of AFm phases. They found no C,AH, or other hydrogarnet phases. Quantitative or semiquantitative determinations of gypsum and ettringite indicated that less than half the total SO, present could be accounted for by these phases. The authors concluded that the solid solution was eventually replaced by a product which they termed 'Phase X', and which was possibly a gel containing all the oxide components of the original cement. Infrared absorption spectra (B97) of unhydrated cements typically show moderate to strong bands at 525 and 925 cm-' due to alite, and at 1120 and 1145 cm-' from S-0 stretching vibrations, and weak bands in the 1650 and 3500 cm- regions due to H,O molecules. The early formation of ettringite
'
202
Cement Chemistry
on hydration is shown by a change in the suiphate absorption to a singlet centred at 1 120 cm- ', and the subsequent replacement of ettringite by monosulphate by a further return to a doublet at l I00 and 1 170 cm- '. The hydration of the silicate phases causes a shift in the broad Si-0 absorption band from 925 to 970 cm-'. After several days, the silicate absorption tends to obscure that from sulphate. Changes also occur in the H 2 0 bending band near 1650 cm- and in the H 2 0 or OH stretching bands at 3 100-3700 cmin the latter region, CH gives a peak at 3640cm-', ettringite one at 3420 cm-' and a weaker one at 3635 cm-', and monosulphate ones at 3 100, 3500 (broad), 3540 and 3675 cm-'. The complexity and partial overlapping of these absorptions lessens their utility. The IR spectra also indicate that both the ettringite and the AFm phases formed in cement pastes are less crystalline than the pure materials, and that the AFm phases become poorer in SO,'- as hydration proceeds.
';
Backscattered electron image of a mature Portland cement paste, aged 2 months. Successively darker areas are of unreacted cement grains (bright), sometimes with visible rims of hydration products, Ca(OH),, other ('undesignated') regions of hydration products, and pores (black). Scrivener and Pratt (S28).
Fig. 7.2
Hydration of Portland Cement
7.1.3
203
Evidence from light and electron microscopy
studies on the microstructures of cement pastes have given results broadly similar to those on calcium silicate pastes (Section 5.3. I), though with some important differences at early ages. As with calcium silicate pastes, the first effective studies were made using light microscopy of thin sections (B51). which were successively augmented by TEM of ground and redispersed material (G4l ,C21), SEM of fracture surfaces (C22,W l8,D 13), X-ray (T19,U12,U13) or backscattered electron (S28,S68) imaging of polished sections, TEM examination of ion-thinned sections (R24) and other techniques. The development of microstructure is considered in Section 7.4. The broad microstructures of pastes more than about 28 days old are similar to those of C,S pastes. Fig. 7.2 shows a typical backscattered electron image, obtained under conditions maximizing contrast from compositional differences. The following types of region are distinguishable by their differing grey levels and by their compositions determined by X-ray microanalysis. (1) The brightest areas are of unreacted clinker phases. Individual cement grains are usually polymineralic and, as in a clinker (Section 4.3. I), the different phases within them can be distinguished by their differing grey levels or by X-ray microanalysis. (2) To varying extents, the clinker phases are replaced by hydration products that have formed in situ, and which appear darker on the backscattered electron image. Most noticeably, the alite and belite are replaced by material approximating in composition to the C-S-H of calcium silicate pastes, but which tends to have a higher Ca/Si ratio and contains small amounts of other elements, especially aluminium, iron and sulphur. Initially, these hydration products are observed as rims on the unhydrated material. As the paste ages, the latter is progressively replaced, until ultimately no significant quantity remains. The in situ hydration products have often been called 'inner product', but this term may be criticized on the grounds that the apparent outlines of the cement grains may lie within the true ones, and 'late product' may be a better description (JIO). Only the larger cement grains, unreacted or otherwise, can be clearly distinguished in this way. (3) CH can be observed as areas darker than the unreacted clinker phases but brighter than the other hydration products. As in calcium silicate pastes, these appear to have grown in regions initially occupied by water. Although the areas appear discrete on two-dimensional sections, they are not necessarily so in the three-dimensional material. They can engulf small cement grains.
204
Cement Chemistry
(4) The remaining space, which forms the matrix in which the above regions appear to be embedded, consists of material varying in grey level from material similar to the in situ product to darker regions, representing pores. Like the CH, these appear discrete in two dimensions, but are not necessarily so in three. This material is often called 'outer product', but the term is imprecise, as it must include not only products formed in what was originally water-filled space, but also that formed in space initially occupied by the interstitial material or by the smaller cement grains, the outlines of which can no longer be distinguished. The term 'undesignated product' is preferable. For most cements, the majority of individual microanalyses of the hydration product in these regions show compositions varying between those of C-S-H and either AFm phase or CH (H4). Regions large enough for accurate microanalysis on polished surfaces ( 2 5 pm) are rarely of pure AFm phase. Transmission electron microscopy of ion-thinned sections provides data at higher resolution than can be obtained with polished sections. Rodger and Groves (R24) described regions which had probably formed in situ from the ferrite phase, and which consisted of C-S-H, a hydrotalcite-type phase and a poorly crystalline phase containing iron that could have been the precursor of a hydrogarnet. The particles of this last constituent were almost spherical and some 20Onm in diameter. The same investigation also showed that much of the product formed in situ from alite or belite was essentially pure calcium silicate hydrate. 7.2 Analytical data for cement pastes 7.2.1
Determination of unreacted clinker phases
The experimental considerations applying to calcium silicate pastes (Sections 5.1 and 5.2) are equally relevant to cement pastes. Of the methods so far used in attempts to determine the degrees of reaction of the individual clinker phases as a function of time, QXDA (C39,D12,T34,P28) has proved much the most satisfactory. Procedures are essentially as for the analysis of a clinker or unreacted cement (Section 4.3.2), but it is necessary to take account of overlaps with peaks from the hydration products, and especially with the C-S-H band at 0.27-0.31 nm. The water content of the sample must be known, so that the results can be referred to the weight of anhydrous material. If a sample of the unhydrated cement is available, and its quantitative phase composition has been determined, it may be used as the reference standard for the individual clinker phases in the paste.
Hydration of Portland Cement
0
10 20 AGE ( D A Y S )
30 0
205
400
Fig. 7.3 QXDA results for the fractions of the clinker phases reacted in Portland cement pastes. Filled circles: Copeland and Kantro (C39), w/c = 0.65. Diamonds: Bezjak et al. (B98), sample C2. Vertical lines: Osbaeck and Jrans (08), range for 7 samples. Open circles: Dalziel and Gutteridge (D 12). Open squares: Patcl et crl. (P28), samples cured at 100% RH. Filled squares: Tang and Gartner (T34), clinker interground with gypsum.
The precision and accuracy of the technique should not be overestimated. If a cement paste contains 50% of alite initially, an error of 3% in the determination of that phase leads to one of 6% in the percentage reaction; if it contains 5% of aluminate phase initially, an error of 1% leads to one of 20% in the percentage reaction. With existing techniques, errors much smaller than these are unlikely to be achieved for untreated pastes. The precision may be greatly increased for the aluminate and ferrite by first removing the silicates by chemical extraction methods (Section 4.3.3). Fig. 7.3 shows some results from the literature. The considerable differences
206
Cement Chem~stry
between the results of different investigations are no doubt partly due to the limitations of the technique, but also to differences between cements or curing conditions, which are discussed in Section 7.7.
7.2.2 Non-evaporable and bound water As with calcium silicate pastes, the gelatinous nature of the principal reaction product renders any definition of chemically bound water some. what arbitrary. The three definitions of water content described in Section 5.2.2 for calcium silicate pastes are relevant to cement pastes, viz. nonevaporable water, chemically bound water and water seemingly essential to the formation of the hydration products in a saturated paste. Water retained after D-drying, known as non-evaporable water, has often been wrongly identified with chemically bound water. It excludes much of the interlayer water in C-S-H, AFm and hydrotalcite-type phases and much of the water contained in the crystal structure of AFt phases. It is often used as a measure of the fraction of the cement that has reacted, but can only be approximate in this respect, because the clinker phases react at different rates and yield products containing different amounts of non-evaporable water. Fully hydrated cement pastes typically contain about 23% of nonevaporable water, referred to the ignited weight. Copeland cJt a/. (C38) determined the non-evaporable water contents of a series of mature cement pastes and carried out regression analyses on the cement composition. For pastes of w/c ratio 0.8 and aged 6.5 years, they obtained the approximate expression: H 2 0 = CaO - 0.5SiO2
+ 5A1203
-
5Fe20, - 2S0,
(7.1)
where H,O represents the quantity of non-evaporable water and all quantities are in moles. The coefficient for SO, was of low reliability, and other sets of data given in the same paper suggest a value nearer to - 1. For a typical cement composition (65% CaO, 21 OO/ SiO,, 5.5% A1,0,, 3.0% Fe,O,, 2.3% SO,), and using a coefficient of - 1 for SO,, this gives a non-evaporable [ water content of 20.4%. r The content of chemically bound water is approximately that retained ond equilibration at I 1 % RH of a sample not previously dried below saturation.1" For fully hydrated pastes of typical cements, it is about 32%, referred t o t ignited weight (F13,T35). There are no systematic data relating this quanti to cement composition. The total content of water essential for comple hydration in a saturated paste is defined as that present in such a pas having the minimum w/c ratio at which complete hydration is possibl
Hydration of Portland Cement
207
(Section 8.2). For typical Portland cements, it is 42-44% referred to the ignited weight.
7.2.3 Thermogravimetry and determination of calcium hydroxide content The discussion of methods for determining C H in calcium silicate pastes (Section 5.2.1) applies also to cement pastes; T G and QXDA are probably the most satisfactory methods. Fig. 7.4 shows a typical T G curve for a nlature cement paste. The step at 425-550•‹C is due primarily to decomposition of CH, and its height, estimated as shown in Fig. 5.3, probably affords the best available method for determining this phase. It is nevertheless subject to at least two sources of error: curves for AFm phases show a step in the same range (Fig. 6.2) and the decomposition of C H is not quite complete within this range (Section 5.1.3). It is probably not practicable in the present state of knowledge to correct for these errors, and doubtful whether either TG or QXDA gives results with an accuracy better than 5 1%.
0
>
-
'
0 . l ~ 50 100
'
I
I
I
3 00 500 TEMPERATURE
n
700
6
a 900
( O C )
TG curve for a Portland cement paste (w/c = 0.5), moist cured at 25•‹Cfor 14 months; full curve, with points calculated as described in the text shown by open circles. Heating rate, 10 deg C min-', flow rate of dry, C0,-free N,, 15 ml n i i n - '; sample weight, 50 mg. Taylor (T5). Fig. 7.4
208
Cement Chemistry
The loss above 550•‹Cis due partly to CO, and partly to the final stages of dehydration of C-S-H and the hydrated aluminate phases. I t is not practicable to distinguish the contributions from TG evidence alone, and, unless evolved gas analysis is used, a separate determination of CO, should be made. As with calcium silicate pastes, serious errors arise if TG determi.. nations are carried out on material that has been treated with an organic liquid, e.g. to stop hydration. Losses above 550•‹Cof more than about 3%, referred to the ignited weight, indicate serious carbonation either from this or other causes. The loss below the CH step is due to decomposition of C-S-H and the hydrated aluminate phases. Although the TG curves of pure AFm phases are markedly stepped in this region (Fig. 6.2), those of cement pastes normally show only slight indications of steps. Weak peaks can, however, sometimes be seen on DTG curves. The absence of steps is probably due to a combination of low crystallinity, the presence of other phases and the presence of AFm phases of different compositions in mixture or solid solution or both. For typical experimental conditions with a 50 mg sample, heating rate of 10 deg C min- ' and N, flow rate of 15 ml min- ', the volatiles retained at about 150•‹C.after correction for CO,, correspond to the nonevaporable water, and those retained at about 100•‹Cto the bound or 11% RH water, but this last temperature, in particular, is very dependent on experimental conditions (T5). For pastes of typical ordinary Portland cements cured for 3-12 months, the CH content found by thermal methods or QXDA is typically 15-25%, referred to the ignited weight (P29,R14,M37,H37,TI7,D12). Pressler et al. (P29) found that for pastes of various ages of ordinary (US Type I ) Portland cements, i t was linearly related to the content of non-evaporable water, but . that for cements high in belite (US Type IV), it tended to a maximum while; the latter continued to increase. This is readily explained. since belite yields 1 only a little CH on hydration. The author has noticed similar behaviour even ) with modern cements high in alite, and that the CH content can possibly;t even decrease slightly after 28-91 days (T5). 1 x4
7.2.4
Determinations of hydrated aluminate and silicate phases
Opinions have differed as to the possibility of determining the hydrate aluminate phases by thermal or X-ray methods. The determination ettringite was d~scussedin Section 6.2.2. Bensted (B99). who used DT found that for ordinary Portland cements the ettringite content increas with time during the first 2 h to maximum values of 2.2-2.8%. and that t quantity of ettringite formed at any given time increased with the w/c rat1
Hydration of Portland Cement
209
There are probably no effective direct methods at present for determining either C-S-H or AFm phases in cement pastes; in both cases, this is probably attributable to the low degree of crystallinity. Odler and AbdulMaula (015) found that determination of AFm phase by QXDA was onlj, semiquantitative. Postulated quantitative phase con~positions of cement pastes may, however, be tested by comparing observed and calculated TG curves (Section 7.3.3). 7.2.5
Analyses of individual phases
The general considerations mentioned in Section 5.2.3 in relation to calcium silicate pastes apply equally to cement pastes. Table 7.1 gives results of X-ray microanalyses of the product formed in situ from the larger alite or belite grains, obtained by analysis of polished sections in the EPMA or SEM. Most of the investigators reported considerable scatter between the results of individual analyses. The reported mean Si/Ca ratios are 0.49-0.60 (Ca/Sl 1.7-2.0). There are indications that the Mg/Ca, AI/Ca and S/Ca ratios arc correlated with those of the cements, or probably more directly with those of the alite or belite that they contain (H4). Rayment and Lachowski (R29) noted a significant dependence on w/c ratio, a 5-year-old paste with w/c = 0.6 giving a unimodal distribution of Ca/Si ratio with a centroid at 1.75. whereas one with w/c = 0.3 gave a bimodal distribution with centroids at 1.75 and 1.95. The bimodularity was due to variations in silicon content, and the distribution of Ca/(Si Al) ratios was unimodal. Apart from a possible decrease in the S/Ca ratios with age (H4), there are no clear correlations with age or temperature. Results for a paste hydrated at 45•‹C(U12) do not differ significantly from those for ones hydrated at lower temperatures. Several investigators have reported X-ray microanalyses of polished sections of the 'undesignated product' formed in space forn~erlyoccupied by water, interstitial material or small cement grains (R28,R29,T19.H4). Fig 7.5 shows a plot of AI/Ca ratios against Si/Ca ratios for individual microanalyses from varying parts of the microstructures of typical Portland cement pastes. Most of the analyses clustering near the lower right-hand corner are of the material formed in situ from the larger alite or belite grains and described in the previous paragraph. The spots analysed were not chosen at random, and CH is much under-represented. The analyses intermediate between those of the in situ product and either CH or AFm phase were, in general, obtained from the undesignated product, suggesting that the latter is largely a mixture on or below a micrometre scale of C-S-H with CH or AFm phase. Rayment and co-workers (R28,R29) obtained broadly similar results, but expressed them by noting a tendency for the ratio of (Ca + Mg) to (Si Al Fe S) to be more nearly constant than that of Ca to Si.
+
+ +
+
Table 7.1 Results of X-ray microanalyses by EPMA or SEM of the gel formed in situ from alite or belite in Portland cement pastesa Na
Mi?
Al
Si
S
tr. n.d. n.d. n.d. n.d. n.d. 0.01 n.d. n.d.
0.05 n.d. 0.02 n.d. 0.03 0.05 0.02-0.08 n.d. n.d.
0.07 n.d. 0.06 0.06 0.08 0.10 0.05-0.07 0.12(?) 0.14(?)
0.59 0.59 0.49 0.60 0.60 0.49 0.50-0.53 0.49 0.53
0.0 1 n.d. 0.02 0.02 0.03 0.01 0.014.03 n.d. n.d.
"Means or other measures of central tendency. expressed as atom ratios relative to Ca. n.d. D~ILB for 6 pastes from 3 cements.
Temp. ("C)
=
Age
Ref.
8 days 28 days 5 years 5 years 23 years 136 years 3-180 days 4 years 60 days
R25 R26 R27 R28 TI9 R27 H4' U 12 u12
not determined or not reported: tr.
=
trace
Hydration of Portland Cement
21 1
P o r t l a n d Cement P a s t e s w l c = 0.5 T = 2S•‹C 28 days 6 months 14 m o n t h s
.
i
Fig. 7.5 Al/Ca atom ratios plotted against Si-Ca atom ratios for individual X-rx! microanalyses of typical Portland cement pastes. Modificd from Harrisson c / ol. W4).
As noted earlier, regions of pure AFm phase of sufficient size to be analysed by EPMA or SEM microanalysis are rare in most cement pastes. Harrisson et al. (H4) found them to be low in iron and sulphur (AI/Ca = 0.50, Si/Ca = 0.05, S/Ca = 0.12, Fe/Ca = 0.01) and that the AFm phase mixed with C-S-H in the undesignated product tended to be higher in sulphur (mean S/Ca = 0.17). The same investigation showed no significant correlations between the Mg/Ca, AI/Ca, S/Ca and Fe/Ca ratios. The authors concluded that little or none of the iron replaces aluminium in the AFm phase, and that magnesium in excess of some quite small amount did not replace calcium in the C-S-H. X-ray microanalyses of one of the old Portland cement pastes studied earlier by Copeland et ul. (C38) confirmed that the principal hydrated aluminate phase was a hydrogarnet and showed its composition to be near Ca,AI,~,Fe,,,SiO,,H, (T19). Studies of ground and redispersed cement pastes by analytical electron microscopy (L28-L30,T17) showed wide variations between individual analyses, even within single particles of micrometre dimensions, and gave mean Ca/Si ratios of 1.5-2.0 for the C-S-H. The mean ratios of minor elements
21 2
Cement Chem~stry
relative to Ca + Mg were broadly similar to those found by X-ray microanalysis, but some particles contained virtually no alum~nium or iron. Particles of AFm phases had lower Fe/AI ratios than the cements from which they were formed, and contained significant proportions of silicon. Parallel studies on calcium silicate pastes gave mean Ca/Si ratios that appear to have been low (Section 5.2.3). Later work, on ion-thinned sections (R24), indicated that the product formed in situ from the cement grains had a Ca/Si ratio of 1.5 0.15. Most of the volumes analysed were found to be pure calcium silicate, but some contained very small amounts of aluminium, sulphur and potassium; no aluminate sulphate was found as a second phase. The 'outer' gel had an average Ca/Si ratio of 1.6 f 0.2 and many areas contained aluminium, sulphur or potassium. The apparent conflicts between these results and those obtained by EPMA or SEM microanalysis have not been resolved.
7.2.6
Silicate anion structure
Silicate anion structures in Portland cement pastes have been studied by the methods described in Section 5.3.2 for calcium silicate pastes. Trimethylsilyshow that, as with lution (TMS) studies (L20,T12,S69,T36,L3I,M43,M44) C,S. the proportion of the silicon present as monomer decreases w ~ t hage and that the hydration products contain dimer, which is later accompanied and eventually partly replaced by polymer ( 35 SI). Some results ha ~ndicatedthat fully hydrated pastes of cement differ from those of C3S that substantial proportions of the s~licateoccur as monomer (S69,L31), but the results of a study in which pastes of C3S, fi-C,S and cement were compared (M44) suggest that the differences between the anion structures of cement and C3S pastes are probably within the considerable experimental errors inherent in the method. The recovery of monomer from unhydrated fi-C,S was only 66% and results for cement pastes can only be considered 5emiquantitative. Another study (L31,M43), in which the T M S derivatives were examined by gel permeation chromatography and other methods, gave the results 4 shown in Table 7.2. There are no significant differences between the distributions of anion size within the polymer fractions of the cement and C,S pastes, and, though the lower totals for the cement pastes impede 4 comparison, probably none between the relative proportions of monomer, dimer and polymer that cannot be explained by kinetics, side reactions or other experimental inaccuracies. In all cases, the data for the molecular we~ghtsof the polymer T M S derivatives, and the mean connectivities of the polymeric anions, are consistent with the hypothesis that the latter are
I
Hvdration of Portland Cement
21 3
linear, with pentamer as the major component and little of the material in &aim containing more than I I tetrahedra. A 29Si N M R study (B100) showed that for cement, as for C3S pastes, the content of Q0 silicate tetrahedra decreases with time and that those of Q' and later of Q 2 tetrahedra increase. After 180 days, the degree of hydration. estimated from the intensities of the N M R peaks, was approximately 90%. These results are consistent with those obtained by the T M S method, and suggest that the hydration products present after 180 days contain at most only a small proportion of monomer. The possible effects on the silicate anion structure of drying, whether during hydration as a result of localized water shortage or subsequently, were considered in Section 5.3.2. Table 7.2 Comparison of silicate anion structures in pastes of C,S and Portland cement (M43)
Starting material: Curing time (months): of totd Si recovered as: Monomer Dimer Polymer Molecular weights of polymer TMS derivatives: Mode Number mean Weight mean Mean connectivity of SiO, tetrahedra in polymer anions:
7.3
Cement 9.5
Cement 13.5
Interpretation of analytical data
7.3.1 The nature of the cement gel
Copeland et al. (C39,C25) treated C,S pastes, and C-S-H prepared in other ways, with various sources of A13+, Fe3+ or SO,2- ions. The XRD peaks of the phases providing the ions disappeared, and changes occurred in the micromorphology of the gels and in the non-evaporable water contents and fractions of the C a O extractable by an organic solvent. It was concluded that the ions were taken up by the C-S-H and that up to about one silicon atom in six could be replaced by aluminium, iron or sulphur, and that aluminium and iron could also replace calcium. These results suggested that the principal hydration product in Portland cement pastes was a substituted CS-H, a conclusion that later appeared to be supported by the results of the
1
21 4
Cement Chem~stry
X-ray microanalyses described in Section 7.2.5. However. none of the evidence shows that the substitutions occur at the crystal chemical level postulated by Copeland et ( I / . , and there are some indications that this is not the case. Stade et (11. (S70) examined C-S-H preparations containing ~ 1 by~ 2 '7 ~ 1 magic angle N M R . At a Ca/Si ratio of 0.95, the aluminium was tetrahedrally coordinated; this is also true of aluminous tobermorite (K34). which is of broadly similar composition and structure. At higher Ca/Si ratios. octahedral aluminium was also present. The fraction of the aluminium in octahedral coordination increased with Ca/Si ratio, and at Ca/Si = 1.5 1 reached unity. The aluminium in pastes made from alite or from C3S with a little C,A was also largely in octahedral coordination (S71). Electron spin resonance and Mossbauer examinations of C-S-H preparations containing Fe3+ showed : that the iron was in octahedral coordination (S72). These results show that the A13+ and Fe3' present in the cement gel do ; not replace Si4', except perhaps to a minor extent. This conclusion is $ reinforced by most of the results of the silicate anion studies described in ,? Section 7.2.6. If there was any significant replacement of silicon at a crystal chemical level, the average anion size and SiO, connectivity would both be $ reduced. Further evidence of a similar nature is provided by the observation ,j that the average anion size in C-S-H preparations made in suspension is not.{ significantly affected by the presence of A13+ (S73). The silicate anion 4 evidence also makes it appear unlikely that there is significant replacement a of silicon by sulphur; it is, in any case, improbable that such replacement: would occur in a dimeric o r chain anion, as a Si-0-S link would be readily hydrolysed. Stade and Miiller (S71) considered that C-S-H of high Ca/Si ratios contained layers of C H structure, in which some of the calcium could be replaced by aluminium or iron. Because of the large difference in size, any such replacement could probably not occur at random, but one calcium atom in three can be replaced in an ordered way, and then gives a layer of AFm structure. This suggests the hypothesis that most or all of the A13+, Fe3' and ions incorporated into the gel are not present as substituents at a crystal chemical level, but in layers of AFm phase intimately mixed with those of C-S-H. Such admixture might occur at various levels, ranging from that of single layers, as in an interstratified clay mineral, t sufficiently coarse for the AFm phase to be detectable by XRD. hypothesis readily accounts for the evidence from X R D and other sources that the AFm phase in cement pastes is poorly crystalline. If this hypothesis is correct, the gel formed in situ from the alite or belite merely a limiting case, in which the proportion of AFm layers is at minimum. This minimum possibly depends on the contents of A13' an
4
Hydration of Portland Cement
21 5
other foreign ions in the anhydrous phases, and the extents to which they tend to pass into the pore solution on hydration. If the C-S--H constituent of the gel is assumed to have the same Ca/Si ratio as in calcium silicate pastes, one would expect that the ratio would be about 1.9 (Si/Ca = 0.53) in a gel with AI/Ca = 0.07. This agrees with some, but not all, of the data in Table 7.1. The hypothesis might explain the observations of Rayment and Lachowski (R29) on the bimodal distribution of Ca/Si ratio in the in situ gel and the relative constancy of its Ca/(Si + Al) ratio. It probably offers the most satisfactory explanation of the existing data but needs to be further tested. Continued studies by TEM of ion-thinned sections may be expected to yield valuable data in this respect.
7.3.2
Fe,O,, SO, and minor oxide components in cement gel
Most of the studies on the hydration of C,AF with or without gypsum (Section 6.5.3) have indicated that the hydration products are AFm, AFt or hydrogarnet phases of higher AI/Fe ratio, together with an iron(II1) hydroxide gel. It has also been suggested that a calcium ferrite gel is formed (B96). There is, however, no experimental evidence that an iron hydroxide gel is formed in cement pastes, and Harchand ct a[. (H38) concluded from a Mossbauer study that it was absent and that the Fe3+ entered the AFm phase. The ferrite phase in cement may well behave differently from pure C,AF, because of the large difference in composition and the effects of other constituents. ) backscattered electron Studies of polished sections by X-ray ( ~ 1 9 and (S68) imaging indicated that the Fe3+ in Portland cement does not migrate through the pore solution on hydration, but remains in products formed in situ. In agreement with this, TEM of ion-thinned sections showed the presence of a poorly crystalline iron-rich phase resembling a hydrogarnet, which was probably formed as part of the in situ hydration product of the ferrite phase (R24). A hydrogarnet high in iron would approximate in composition to the calcium ferrite gel suggested on other grounds. It could accommodate in its octahedral sites not only A13 and Fe3 +,but also Ti4+ and Mn3+, which are concentrated in the ferrite phase of the clinker. In an ordinary Portland cement, only some two-thirds of the Fe3 occurs in the ferrite, the rest being contained largely in the alite and aluminate (Table 4.3). On hydration, the Fe3+ in these other phases probably does not enter a hydrogarnet, but goes into AFm phases or layers formed in situ. This would account for the observation by analytical electron microscopy that small amounts of Fe3+ are present in the AFm phases. One would expect that any relatively large crystals of periclase in a clinker +
+
21 6
Cement Chemistry
would hydrate to give brucite. However, in modern clinkers. most of the Mg2+ is not present in this form, but in the major clinker phases, especially alite and ferrite, and, if the MgO content exceeds about 1.5%, in small crystals of periclase intimately mixed with aluminate and ferrite in the interstitial material. In all these cases, it is associated with comparable 0, larger amounts of A13+,and one would then expect that a hydrotalcite-type phase would be formed. TEM and X R D evidence (Sections 7.1.1 and 7.1.3) indicate that this is the case. M g Z f , like Fe3+, does not readily migrate through the pore solution, and the formation of a hydrotalcite-type phase is readily explained by the substantial magnesium content of the ferrite. A hydrotalcite-type phase formed from the Mg2+ and part of the A13+ present in the alite or belite could be mixed with C-S-H in the manner suggested for AFm phase. Sulphate present in the gel could be accommodated as interlayer anions associated with AFm layers. C-S-H can, however, take up CaSO, in the absence of A13+ or Fe3+,and thus of AFm layers. Odler (017) showed that CaSO, thus bound was very readily removed, and suggested that the Ca2+ and ions were adsorbed. The K + and Na+ ions that are found in the cement gel by X-ray microanalysis are partly the result of deposition from the pore solution when the latter evaporates during specimen preparation or in the high vacuum of the instrument, and these, too, with equivalent amounts of OH-, may be adsorbed; after deducting the fractions deposited from the pore solution, the K/Ca and Na/Ca ratios in the gel are typically -0.0 1 and <0.01, respectively (Section 7.5.2). The considerations discussed in this and the preceding section suggest the cement gel is a complex material, comprising several types of s constituent together with the pore solution. Most of the solid constituen viz. the C-S-H, AFm and hydrotalcite-type phases, have layer structur and the layers are probably mixed on scales varying from the nanometre the micrometre level. Hydrogarnet and AFt phase, neither of which has layer structure, could also be mixed with the other constituents on or be1 a micrometre scale. The relative proportions of the different solid con tuents vary in microstructurally different regions. If this description correct, C-S-H describes only one, or more probably two, of the s constituents, albeit the most important. Provided that this is recognized, term 'C-S-H gel' may still be considered appropriate for the materi alternatively, the term 'cement gel' is possibly to be preferred.
7.3.3 The stoichiometry of cement hydration Many attempts have been made to formulate the stoichiometry of cement
Hvdration of Portland Cement
21 7
hydration (e.g. P30,T17,T35). The following treatment incorporates features of those in the references mentioned. It applies t o ordinary Portland cements hydrated in pastes of w/c ratio 0.4-0.6 at 15-25•‹C for times greater than about I day. Modifications would be needed for other curing conditions or other types of cement, such as sulphate-resisting cements. The essential input data are (a) the bulk chemical composition of the cement, (b) the quantitative phase composition of the cement and the &emical compositions of its individual phases, (c) the fraction of each phase that has reacted, (d) the w/c ratio, (e) the CO, content of the paste and an estimate of how it is distributed among phases, and (f) the composition of each hydrated phase for the specified drying condition. If (b) is unknown. it may be estimated as described in Section 4.4, and if (c) is unknown, i t may be estimated from the age as described by Parrott and Killoh (P30), or, more simply though less precisely, by using empirical equations (D12,T37). If the phase composition by volume and porosities are to be calculated, densities of phases are also required. In the initial stages of the calculation, quantities of elements are best expressed as millimoles per 100 g ignited weight. The hydration products of the ferrite phase are first considered; it is assumed that all the magnesium released from it on hydration, with an equivalent amount of its aluminiunl, enters a hydrotalcite-type phase and that all the iron, silicon, titanium and manganese released from it, with some of the calcium and aluminium, enter a hydrogarnet-type phase. The remaining calcium and aluminium from the ferrite are pooled with the atoms supplied by the other cement phases. I t is then assumed that the magnesium, with an equivalent amount of the octahedral cations (aluminium, titanium, manganese and iron), enters a hydrotalcite-type phase. The silicon is assumed to occur as C-S-H, and by solving two simultaneous equations using the amounts of sulphur and octahedral cations, the amounts of A F m and AFt phases are obtained. This procedure can be modified, if desired, to place some silicon in the aluminate phases, or aluminium in the C-S-H or both; three equations will then have to be solved. Calcium not present in any of the phases thus far considered. or in CaCO,. is assumed to occur as C H . On this basis, a table is constructed giving the amount of each element present in each phase, expressed in millimoles per I00 g ignited weight. For any selected drying condition, a table is now completed, similar to the previous one, but with the H,O and CO, content of each phase included and the quantities of elements expressed as weight percentages of oxide components. Table 7.3 shows results for a typical, mature cement paste thus calculated. It was assumed that the C-S-H had Ca/Si = 1.7 and AI/Ca = 0.02, that the AFm and AFt phases each had Si/Ca = 0.05 and that the hydrogarnet-type phase had the composition Ca AI,,,Fe Ti, ,Mn,
,,,,
,,,,
,,-
Table 7.3 Calculated mass balance and compositions by weight and by volume for a 14-month-old cement paste (w,k
=
0.5)"
Prvcetztages on rlze igrzired w e i g h ~ f o rnraret-id equilibrated at 11% RH N a 2 0 MgO Alite Belite Aluminate Ferrite
C-S-H Ca(OH), AFm AFt
Total
A1,0,
SiO,
SO,
K20
CaO
TiO,
M n 2 0 , F e 2 0 , Other CO,
H20
Total
Volume percentuges for paste equilibruteu' ut I I% RH Alite
Belite
Alum.
Ferr.
C-S-H
CH
AFm
AFt
FeHP
M g HP C a C O , O t h e r
Pores
Wurrr contents (prrcentnges ot7 the ignited nvight) B o u n d water: 31.2% Non-evaporable water: 21.9% Porositiesc Capillary. 15.5%
F r e e water. 27.0%
T o t a l water, 41.9%
" Modified from Ref. T35. 'Other' components mainly P,O, (0.2%); other phases mainly alkalis (adsorbed or in solution). insoluble residue and P,O, (form of combination unknown). Fe H P = hydrogarnet-type product from ferrite phase. Mg H P = hydrotalcite-type phase. AFm and Mg H P include poorly crystalline materials intimately mixed with C-S-H. Discrepancies in totals arise from rounding. Free water porosity. 'Porosities defined in Section 8.3.1.
220
Cement Chemistry
Table 7.4 TG data, water contents and densities for phases In Portland cement pastes
-
Phase and mole ratio of volatiles retained at pven temperaturea Temperature (
c)
1 .00 1.00
3.00 2.80
5.20 4.50
1.76 1.76
2.67 2.49
1 .OO 1 .OO
Saturated 11% RH D-dry
2.30" 1.20 0.85
1.00 1 .OO 1 .OO
3.50 3.00 2.00
5.33 5.30 1.20
1.76 1.76 1.76
2.50 2.50 1.90
1 .OO 1 .OO 1 .OO
Saturated IIO/o RH D-dry
1900" 2180 2700
2240 2240 2240
1900 2010 2400
1770 1730 2380
3100 3100 3100
2000 2000 2300
2710 2710 2710
Ahte, 3150
-
C-S-H C H AFm AFt F e H P h M g H P c Any H 2 0 / C a H 2 0 / C a H 2 0 / C a H,O/Ca H 2 0 / C a H 2 0 / M g C 0 2 / c a
Belltc, 3300
Aluminate, 3060
"TG wlth he'it~ngrdte 10degC mln-I, 50mg
H~ydrog~irnet-type product from ferrlte phdse ' Hydrotalclte-type phase. '' H20/Mg In case o f Mg H P ' Of unccrtdln slgnllicdnce for w/c > 0 4 For typ~c'ilcornposltlons
specimen.
Ferr~te,3570
tlow rate of dry N, 15 ml nun-'.
d t
Hydration of Portland Cement
221
Si,,,O,,H,,,,. Table 7.4 gives other data assumed. By introducing also the w/c ratio and the densities of the phases, the volume percentages of phases for the specified drying condition may now be calculated, and, by difference, the porosity. Using data for the T C curves of the phases containing H,O or CO, or both, one may also calculate a simulated T G curve for the paste. The results of such a calculation are given in Fig. 7.4, the observed curve in which relates to the 14-month-old paste of Table 7.3. Results such as those in Table 7.3, and calculated TG curves such as that in Fig. 7.4, agree broadly with experimental data, but many of the data assumed are tentative and the method needs to be tested in many more cases before its accuracy can be assessed. The hydration products listed. other than CH and, in part, the AFm and AFt phases, are probably not separate phases on more than a sub-micrometre scale, but constituents of the cement gel. If the data were available, it might be more meaningful to group the volume percentages differently, by such categories as in situ products formed at the various stages of hydration from each of the clinker phases, and undesignated product, and to give the chemical and 'phase' compositions for each. AFm phase as defined in Table 7.3 represents a weighted mean for products of differing compositions and states of subdivision and crystallinity, ranging from relatively well defined crystalline material, probably low in SO,, to material intimately mixed with C-S-H and probably higher in SO,. A similar comment may apply in some degree to AFt phase.
7.4 7.4.1
Development of microstructure The early period of hydration
Many features of the development of microstructure in cement pastes are similar to those observed in C,S pastes (Section 5.3.1), but there are some ~mportantdifferences. As with C,S pastes, it is convenient to consider the process in terms of early, middle and late periods of hydration. with divisions at approximately 3 and 24 h after mixing. Fig. 7.6 shows diagrammatically the sequence of changes undergone by a typical, polymineralic cement grain, as given by Scrivener, on whose conclusions (S39,S68) the following is largely based. Studies by HVTEM show that at high dilutions a gel layer or membrane forms over the surfaces of the grains soon after mixing (D14,C43). A product of similar appearance has been observed in pastes of C,A with gypsum (S41), and, less distinctly, in pastes of cement of normal w/c ratios (523.S68). It is probably amorphous, colloidal and rich in alumina and silica, but also containing significant amounts of calcium and sulphate, the composition varying with that of the underlying surface. Within about 10 min.
Hydration of Portland Cement
223
stubby rods of AFt phase are also seen (D25SD27). They are typically some 250 nm long and 100 nm thick. Studies using wet cells show them to occur both on the surfaces of the grains, and at some distance away (S41,S68) (Fig. 7.6b). They are probably more abundant near to the surfaces of the aluminate phase, and appear to nucleate in the solution and on the outer s~~rface of a layer of gel. On drying, this layer shrinks, and the AFt crystals fall back onto the surfaces of the cement grains. The early products thus ditTer in morphology and composition from the exfoliating foils or honeycombs of C-S-H that have been observed in C,S pastes. The first study using a wet cell, made at high w/c ratios, showed tubular growths radiating from the cement grains, which were considered to have formed by a 'silicate garden' mechanism (D14). Later work showed that they were rich in calcium, aluminium and sulphur, and that they did not form if C,S was substituted for cement (B101). They have not been observed in the more recent studies made at normal w/c ratios, and do not appear to be a significant feature of normal cement hydration.
7.4.2 The middle period of hydration During this period, which begins at about 3 h and ends at about 24 h, some 30% of the cement reacts. I t coincides with that of strong heat evolution and is characterized by the rapid formation of C-S-H and CH: Studies using wet cells show that the undried C--S-H has a filmy, foil-like morphology (P31,S40), which on drying changes to give fibres ('Type I C-S-H') where space is freely available or honeycombs or reticular networks ('Type I 1 C-SH') where it is more restricted (Section 5.3.1). The morphology is also affected by the presence of foreign ions, such as CI-. Spherical aggregates of fibres some 2 pm in diameter, observed in dried material, probably result from the rapid reaction of small cement grains consisting only of alite (S68). The CH forms massive crystals in the originally water-filled space. Nucleation sites appear to be relatively few in number, and the growing crystals may engulf some of the smaller cement grains. All these features of the process are similar to those observed in C,S pastes. The C-S-H forms a thickening layer around the cement grains (D26,P3 1 ) which engulfs and perhaps nucleates on the AFt rods (Fig. 7 . 6 ~ ) .A significant amount has formed by 3 h, and the grains are completely covered by 4 h. The shells grow outwards; by about 12 h they are some 0.5-1.0 pm thick, and those surrounding adjacent grains are beginning to coalese. At this stage, which was called the cohesion point, fracture through the shells begins to supplant fracture between them. It coincides with the maximum rate of heat evolution and corresponds approximately to the completion of
224
Cement Chemistry
setting. The structure of interconnected shells has been considered to play an important part in determining the mechanical and other properties, which thus depend on the particle size distribution of the cement (D26). Studies using ion-thinned sections, wet cells and backscattered electron images of polished sections show that a space develops between the shell and the anhydrous material (S40,S41,S68) (Fig. 7 . 6 ~ ) In . this respect, the hydration of cement differs from that of C,S, in which the C-S-H grows directly over the C,S surfaces, without any detectable separation (S41). By 12 h, the spaces are up to 0.5 pm wide. They are likely to be filled with a highly concentrated or colloidal solution, and the shells are evidently sufficiently porous at this stage that ions can readily migrate through them (S68). The existence of spaces shows that reaction proceeds by dissolution and precipitation; further evidence for this is provided by the fact that the C-S-H also deposits on the surfaces of pfa particles, if these are mixed with the cement (D28). Some other relatively unreactive or inert admixtures behave in the same way. Towards the end of the middle period, a renewed growth of AFt crystals takes place (D27,P31 ,S41) (Fig. 7.6d). They are markedly more acicular than those formed earlier; their lengths are typically 1-2 pm, but sometimes up to 10pm. Their formation is associated with a shoulder on the heat evolution curve (Section 7.5.1). Their formation implies an increase in the rate of reaction of the aluminate, or less probably the ferrite phase. which is probably related to the reaction of the alite (S68).
7.4.3 The late period of hydration With the decreasing permeability of the shells, C-S-H begins to deposit also on their insides, and its surface advances inwards more quickly than that of the alite retreats. Grains smaller than about 5 pm appear to react con~pletely before the end of the middle period, and before much material has deposited inside the shells; many that originally contained aluminate phase are empty (S68). The outer product from such grains is often absorbed i n the shells surrounding adjacent, larger grains. With larger grains, the spaces between shell and core fill up, and by about 7 days they have disappeared; at this stage, the shells are typically some 8 pm thick and consist mainly of material that has been deposited on their inner surfaces (Fig. 7.6e and f) (S68). The separations between shell and core seen on polished or ion-thinned sections rarely exceed about 1 pm. Separated shells, up to at least 10pm across and sometimes completely
Hydration of Portland Cement
225
hollow, were originally observed on fracture surfaces ( 8 102) and have becn called 'Hadley grains'. These are not observed on ion-thinned sections. and possibly result from the core falling out during specimen preparation (S68). It would be difficult to reconcile any widespread occurrence of such caiities in a mature paste with the bulk density of the material (P31). The concentration of SO,'- must drop rapidly inside the shells a s the aluminate phase reacts, and AFm phase often forms within the shells. any AFt phase formed there initially being replaced by AFm as a result of continued reaction of the aluminate phase (S41 ,S6X). A single spccimcn may show AFm phase within the shells and AFt phase outside them. The XRD evidence (Section 7.1.1) shows that significant quantities of AFt phase may persist, apparently indefinitely; this is presumably material that has becn precipitated outside the shells. If the S O , ' concentration in the solution outside the shells drops before these have sufficiently isolated the anhydrous grains, relatively large ( - 10 pm) crystals of AFm phase can form throughout the paste (S68). After the spaces between shells and cores have filled up, reaction is slow and, in contrast to that occurring earlier, appears to occur by a topochemical mechanism (Section 5.6.4). In old cement pastes, three regions of C-S --Hcan thils be distinguished in the relicts of the larger fully reacted grains (S74). I?:. ( a ) an outer layer some I pni thick that has formed through solution in originally water-filled space, (b) a middle layer some 8 pm thick that has been deposited, also through solution. on the inside of the shell and thus in space originally occupied by the cenient grain, and (c) a central core, that has formed topochemically. As with C,S pastes, all the C-S-H probably has a foil morphology, which is modified in varying ways by the erects of drying and in accordance with the amount of space available. Except in pastes many years old. the hydration of belite has proved difficult to observe by electron microscopy (S68), although X R D evidence shows that considerable reaction has occurred within 14 days. In a 23-!.earold paste studied by backscattered electron imaging, the belite grains had reacted completely. but the original lamellar structure remained, providing further evidence of a topochemical mechanism (S74). The belite appears to react preferentially along the exsolution lamellae. The reaction of the ferrite phase has similarly proved difficult to observe, though backscattered electron imaging showed the formation in the same sample of rims high in iron. and TEM examination showed the formation of the in situ product containing C-S-H, hydrolalcite-type material and poorly crystalline iron-rich material mentioned in Sections 7.1.3 and 7.2.5.
226
Cement Chemistry
7.5 C a l o r i m e t r y , p o r e s o l u t i o n s a n d e n e r g e t i c s 7.5.1
The early and middle periods
Fig. 7.7 shows the heat evolution curve for a typical Portland cement paste, determined by conduction calorimetry. It broadly resembles those given by C3S (Fig. 5.14) and comparison with the microstructural evidence (Section 7.4) shows that it can largely by explained in a similar way. The initial peak ( 1 ) is attributable to a combination of exothermic wetting and the early. stage reactions, which with cement give a gelatinous coating and rods of AFt phase. Rehydration of hemihydrate to give gypsum may contribute. The main peak (2) corresponds to the middle-stage reaction, in which the main products, as with C3S, are C-S-H and CH. The gradually decreasing rate of heat evolution after 24 h corresponds to the continuing slow reactions of the late stage, which again give mainly C-S-H and CH. AS with C3S, the process may be divided into the five stages of initial reaction, induction, acceleratory and deceleratory periods. and the final period of slow reaction.
z z OO
I
10
20
TIME
30
40
50
(HOURS)
Fig. 7.7 Rate of heat cvolution at 20•‹Cfor a typical Portland ccmcnt; for significance of numbered peaks, sec text. After Pratt and Ghose (P31).
Many cements show a shoulder or more definite peak (3) at about 16 h. This has often been associated with the conversion of AFt into AFm phase, but comparison with the microstructural evidence shows that this is incorrect (P31); it is associated with the renewed formation of ettringite. A further, less distinct shoulder (4) has been associated with hydration of the
Hydration of Portland Cement
227
ferrite phase (P31) or with the conversion of AFt to AFm phase (S68). I t may correspond to a peak at about 50 h observed by Stein ( ~ 7 5 ) In . all these reactions involving the aluminate or ferrite phases, the principal exothermic component is probably the reaction of the anydrous compound with water and not the precipitation or subsequent reactions of hydrated compounds. Many studies of concentrations in the pore solution during the early and middle periods have been reported (L32,L33.T38,U12,G58). Fig. 7.8 shows typical results. Relatively high concentrations of C a 2 + , K', Na'. SO,'and OH- are quickly reached. Between 2 and 12 h in the example given, the concentrations change relatively little, indicating an approximate balance between the continued dissolution of the cement phases and the precipitation of products. At 12-16 h, the concentrations of C a 2 + and SO,'- fa11 sharply, and the solution thereafter is essentially one of alkali hydroxides. The sharp fall at 12-16 h corresponds to the renewed growth of ettringite observed in the SEM and to shoulder (3) on the heat evolution curve.
0
10 TIME (HOURS)
20
Fig. 7.8 Concentrations in the pore solution (scaled as indicated in the cases of OH- and SiO,) of a Portland cement paste of w/c ratio 0.5. After Lawrence (L32).
228
Cement Chemistry
Concentrations of SiO, and AI,O, are low throughout, values of 0.030.05 mmol 1 - ' for Si and below 5 ppm for Al having been reported for times from 20 s to 45 min (T38). Any more detailed interpretation demands that both activity coefficients and the existence of complex species be taken into account (B90.G58). Gartner P I (11. ((358) showed that, for most cements, the solution is saturated in CH within 12 min and that the saturation factor (defined as the activity product divided by its value at saturation) reaches a maximum of between 2 and 3 within 2 h. For gypsum, saturation is reached within 6min. but the saturation factor never exceeded 1.3 in the cases studied. With cements high in K,O, the solution can become saturated in syngenite. Gartner ct 01. found that 'the K and Na concentrations increased rapidly during the first 12 min and only slowly, or not at all, during the subsequent period up to 3 h. Other methods that have been used to follow the reactions in cement pastes during the first day include measurements of AC electrical conductivity (T39,A14) and dielectric constant (A14). One may calculate a partial mass balance for the reaction at an age of 30 niin. QXDA of a typical cement retarded with gypsum showed that, at this age, I I% of the aluminate and 8% of the ferrite phase had reacted (T34). Assuming from the bulk analysis that the cement contained I"% of nluniinate and 7% of ferrite, and that these phases had the typical conipositions given in Table 1.2. a total of 1.Og of CaO and 0.5 g of A1,0, are released from them per 100 g of cement. For broadly similar cements at the same age. ettringite contents of about 2% have been typically reported (B99.UI3). These appear to be referred to weights of paste. and suggest values of about 3% referred to that of cement. This quantity of ettringite, ignoring any iron substitution, contains 0.8 g of CaO. 0.2 g of AI,O, and 0.6 g of SO,. It thus appears that, of the A1,0, released, less than half enters the ettringite. Apart from small amounts in solution, the remainder and the CaO, SiO, and part of the SO, released from other phases presumably enter the amorphous gel that is also formed. The evidence from microstructure, calorimetry and other sources suggests that the hydration processes of cement and C,S are essentially similar. There are important differences in the nature of the early product and in where the C-S-H formed in the middle stage of reaction begins to deposit. but in both cases it would appear that the early reaction slows down because of the deposition of a layer of product, which either isolates parts of the anhydrous surfaces from the main solution or allows the concentrations close to those surfaces to rise to values approaching the theoretical solubilities of the anhydrous compounds. In both cases, the initiation of the main reaction and the kinetics in its acceleratory phase appear to be controlled by the nucleation and growth of C-S-H. +
+
Hydration of Portland Cement
7.5.2
229
Pore solutions after the first day
Apparatus for extracting pore solution from hardened cement pastes b! applying pressures of some 375 MPa was first described by Longuet c t (11. (L34). and many investigators have reported analyses of solutions thus obtained (L34,D29,X I ; for further references, see Ref. T37). With pastes of normal Portland cements more than about I day old, the only ions present in concentrations above a few mmol I - ' are K + , Na' and O H - . Most studies show the concentrations of these ions to rise with time, and to approach a limit after 28-90 days. Some show concentrations passing through maxima, with subsequent decreases that are usually slight. Typical concentrations after 180 days for pastes of w/c ratio 0.5 are 0.08 mol I - ' for Na' and 0.24 rnol 1 - ' for K + for a low-alkali cement (0.16% Na,O, 0.43% K,O) (XI) and 0.16 rnol I for Naf and 0.55 rnol 1 for K' for a highalkali cement (0.24% Na,O, 1.21% K 2 0 ) (L34). The corresponding OHconcentrations are 0.32 rnol I - ' and 0.71 rnol 1 -', respectively. Some investigators (e.g. Ref. L35) have reported concentrations varying with the pressure used to express the liquid, suggesting the possibility that high concentration gradients exist and casting doubt on the interpretation of the results. This question does not appear to have been resolved. The K i and Na' are present in the cement partly as sulphates and partlj. in the major clinker phases (Section 3.5.6). When the phases containing them react, the accompanying anions enter products of low solubility and eqtrivalent quantities of O H - are produced. The K t , Na' and OH- ions arc partitioned between the pore solution and the hydration products. Early estimates of the fractions remaining in solution were too high because nonevaporable water was used as a measure of bound water and the quantity of pore solution thereby overestimated. Taylor (T37) described a method for predicting the concentrations at any desired age after I day from the w/c ratio and the contents of total Na,O. total K,O, water-soluble N a 2 0 and water-soluble K 2 0 in the cement. I t \\.as assumed that the amount of each alkali cation taken up by the products is proportional to its concentration in the solution and to the quantit!. of products (C-S-H and AFm phase) taking it up. This led to the equation
-'
c
=
m,/[V
+ (h x
P)]
where c is the concentration in rnol I - I , m, the quantity in rnillimoles of alkali cation released, V the volume of pore solution in ml, P the quantity of relevant products (C-S-H AFm phase) relative to that formed on complete hydration, and b an empirical constant, called the binding factor, equal to 20.0 ml for K and to 3 1.0 ml for Na +.All quantities are referred to 100 g
+
+
230
Cement Chemistrv
of cement. V was taken to be the total volume of water less the volume of bound water, i.e. that retained at I I % RH. The latter quantity. and t n r and P, were estimated using empirical equations. For a Portland cement with 0.2% total Na,O and 0.6'%, total K,O, at w/c = 0.5 and an age of I year, calculation by the above method indicated that 59% of the Na+ released and 48% of the K + released were taken up by the products. The ratios of alkali cations to C a 2 + in the products are sufficiently low (typically < 0.01 for Na' and -0.01 for K + ) as to suggest that the ions, with an equivalent quantity of O H , are adsorbed. As Glasser and Marr ((363) concluded earlier, N a + appears to be more strongly held than K + . As was noted in Section 7.3.2, the ratios of alkali cations to CaZ+ found in the cement gel by X-ray niicroanalysis are higher than those in the solid products, because they include the materials dissolved in the pore solution, which are deposited when the latter evaporates. 7.5.3
Energetics of cement hydration
Copeland et ul. (C38) reviewed the energetics of cement hydration. After the first few days, the rate of heat liberation is too low for conduction calorimetry to be a practicable means of investigation, but the total amount of heat liberated after any desired time can be determined from the heat of solution in acid, which is compared with that of the unhydrated cement. Fig. 7.9 gives average results thus obtained for different US types of Portland cements. 1 I I I
----DAYS ----- YEARS-AGE (LOG SCALE) Average cumulative heat evolution for a total of 20 Portland cements of US Types I, 11, 111 and IV, hydrated at w/c = 0.4 and 21•‹C.After Copeland et a/. (C38). Fig. 7.9
Hydraf~onof Portland Cement
231
With any Portland cement, the amount of heat evolved by a given time is directly related to the amounts of the clinker phases that have reacted. and thus depends on such factors as the particle size distribution, w/c ratio and temperature and R H of curing. As the results in Fig. 7.9 show. i t also depends on the cement composition. At all ages, the anlount of heat liberated is greatest for thc Type Ill (high early strength) cements and least for the Type IV (low heat) cements. The differences reflect the ditl'crent Liverage potential phase con~positions, which show progressively lower contents of C,S and C,A. and progressively higher ones of CIS, along the sequence from Type I l l to Type IV. Using least squares regression analysis, the heat liberated at any given age was predicted from the potential phase composition by equations of the type
where H, is the anlount of heat evolved in kJ kg-', the formulae denote thc weight fractions of the phases calculated by the Bogue equations and the coeficients ( I . h. i ~ n d(/ have the values given in Table 7.5, which also includes values for the enthalpies of hydration of the pure clinker compounds, taken from the data in Sections 5.5.3 and 6.5.4. The agreement between the least squares coefficients and the enthalpies of hydration is good for mature pastes, but there are some anomalies for younger pastes. In principle. it should be possible to calculate the heat of hydration from the quantitative phase compositions of the unreacted mix and of the paste. using standard enthalpies of formation. The sensitivity of such calculations to small errors in the latter data probably renders this approach unsatisfactory with existing data.
7.6 A c t i o n s o f c a l c i u m s u l p h a t e a n d o f a l k a l i s 7.6.1
Setting
Setting times are commonly defined empirically, using a Vicat needle. In this device, weighted needles of standard design are allowed to sink into the paste, and initial and final set defined as the times when the degree of penetration falls below specified levels. In the British standard, which is typical, initial set must occur not earlier than 45 min, and final set not later than 10 h. Some workers have concluded from QXDA evidence that the rate of consumption of the aluminate phase is not significantly affected by the presence of gypsum (L33,L36,0 IS), and setting has been attributed largely to
Table 7.5 Coefficients in equation 7.3 for predicting the cumulative heat evolution in a cement paste of given age from the potential phase composition of the cement (for w/c = 0.4 and 21•‹C)(C38) Value of the coefficient (kJ kg-') for age given below Compound Coefficient
3 days
7 days
28 days
" F r o m data in Chapters 5 and 6. * Reaction with gypsum to give C , A ~ H , , . ' Reaction with gypsum to give ettringite. "Ucaction in prcsence of cxcess CII to give a hydrogi~rnct.
90 days
1 year
6.5 years
13 years
Enthalapy of complete hydrationa (kJ kg-')
Hydration of Portland Cement
233
recrystallization of ettringite (L33). In contrast, Tang and Gartner (T34) found that the amounts of both aluminate and ferrite phases reacting during the first 30 min were substantially decreased in the presence of gypsum or other forms of calcium sulphate, and evidence from XRD, DTA, IR spectroscopy, conduction calorimetry and SEM shows that setting occurs during the acceleratory period and thus corresponds to a period of rapid formation of C-S-H and CH (B103). The microstructural evidence (Section 7.4.2) shows that little change occurs in the amount o r morphology of the ettringite during this period, the secondary growth of ettringite occurring later. The concept of a cohesion point, at which the shells surrounding individual cement grains coalesce (D26). further indicates the importance of the relation between C-S-H formation and both setting and the initial stages of strength development. The close similarity in setting behaviour between pastes of cement and C,S, and the relation between setting and microstructural development in C,S pastes, provide further evidence that normal setting depends primarily on the silicate reactions. If too little gypsum is added, or none at all, many cements undergo what is termed 'flash' or 'quick' set. This is a rapid set, with much evolution of heat; plast~cityis not regained on continued mixing, and the subsequent development of strength is poor. It is associated with increased early reaction of the aluminate and ferrite phases (T34), and with the formation of plates of AFm phase throughout the paste. These can be 5-10pm in size (L36). The meshwork of plates, which may be contrasted with the much more compact coatings of gel and small rods of AFt phase formed in the presence of an adequate supply of gypsum, accounts for the rapid stiffening, but the reason for the subsequent poor strength development is less clear. Possible explanations include the formation during the initial reaction of strongly protective layers of product around the cement grains, and the existence of a microstructure weakened by the large plates of AFm phase. Some cements low in aluminate phase d o not show flash set even if no gypsum is added. Another undesirable condition is called 'false set'. This is also a rapid set, but there is no abnormally high evolution of heat, plasticity is regained on further mixing, and subsequent strength development is not markedly affected. The most usual cause of false set is the presence of too much calcium sulphate in the form of hemihydrate, which is rehydrated to give gypsum (called 'secondary' gypsum). The setting is attributable to the interlocking of the gypsum crystals, which are tabular and can be 5-10 pm In longest dimension (L36). If the quantity of secondary gypsum is not too great. it redissolves on further mixing, and the reactions then follow their normal course. Dehydration of gypsum to hemihydrate can occur during milling; hemihydrate may also be added deliberately, if a more reactive form of calcium sulphate is required. With cements high in K,O, syngenite can be
234
Cement Chemistry
precipitated; this also can cause false set, and, due to thc removal of CaSo, from the system, even Rash set (524).
7.6.2 Optimum gypsum The gypsum in cement affects not only the setting time. but also the strength development and the volume stability. Locher cJ/ uI. (L33.L37). Kinare and Gartner (K35) and Tang and Gartner (T34) have discussed factors governing the optimum content and the effects of varying the source of the sulphate. The situation is complicated by the fact that. contrary to some wrly conclusions, the amounts needed to optimize different properties, such as strength at various ages and drying shrinkage, are not necessarily the same; also, the amount needed to optimize a given property in a concrete may not be the same as that required in a paste or mortar (K35). During the early and middle periods of reaction in a cement paste, gypsum or other forms of calcium sulphate dissolve and react at or close to the surfaces of the clinker grains, o r more specifically those of the aluminate and ferrite phases. The factor most directly influencing the course of the early reactions is not so much the relative amounts of calciunl sulphate, aluminate and ferrite phases, as the rates at which the relevant ionic species are made available a t the surfaces of the cement grains. The rates at which Ca2+ and SO,'- ions are supplied by the calcium sulphate thus depend both on the amount of the latter and on its physical and chemical nature. Hemihydrate or soluble anhydrite supply ions more quickly than does gypsum. which in turn supplies them more quickly than anhydrite. Other major factors affecting the supply of these ions are the particle size distribution of the calcium sulphate and the distribution in space of the particles; Tang and Gartner (T34) found that a given proportion of gypsum added by intergrinding was much more effective than the same proportion added by blending, all other conditions being as nearly as possible equal. Intergrinding presumably brings the gypsum particles into more intimate contact with those of the clinker. Increase in any of the factors rendering the CaSO, more readily available will tend to decrease the amount that is needed to produce a given effect in the early stage of reaction. Increase in temperature has a similar effect (C39). Factors affecting the rate at which Al,O, is supplied include its content in .: the clinker, its distribution among the clinker phases and especially the j amount present in the aluminate, the specific surface area of the ground clinker, the reactivities of the aluminate and ferrite, and the microstructure j of the clinker particles, or, more specifically, the areas of surface composed 4 of aluminate and ferrite phases and the manner in which these phases are,]
Hydration of Portland Cement
235
intergrown with each other and with the silicate phases. Any factor increasing the availability of the A1,0, will tend to increase the amount of gypsum that is required to produce a given effect in the early reactions. Sulphate ion is also supplied by the clinker, especially as alkali sulphates or calcium langbeinite, and C a 2 + ions are supplied by the clinker phases. including free lime. The alkali sulphates provide a highly available source of SO,?-, but the alkali cations, or more probably the O H - ions that they ~ r o d u c e ,have additional effects (Section 7.6.3). The minimum content of SO, required to control setting is typically around 2%. Increase in SO, content beyond the minimum required has little effect on setting unless the proportion of hemihydrate or soluble anhydrite is so high as to cause false set. It causes a progressive decrease in the shrinkage that occurs in dry atmospheres (e.g. at 50% RH) and a progressive increase in the expansion that occurs in water. Hobbs (H39) gave data for cements containing more than the normal amounts of gypsum. Mechanisms of expansion are discussed in Sections 10.2 and 12.5. If expansion is excessive. the concrete disintegrates; this limits the permissible SO, content, which. in typical specifications, is set a t 2.5-3.0%, depending on the potential C,A content of the clinker. The effects on strength are complex and not well understood. In general, for any particular clinker and source of SO, there is an optimum content of SO, at early ages. In a careful study, in which strengths were corrected empirically for the effects of differing particle size distributions and contents s found maximum I-day strengths at of entrapped air, Osbaeck and J ~ n (08) 3-5% SO, for a number of cements having different alkali contents and t s been clinker S03/alkali ratios. At later ages, somewhat variable r e s ~ ~ lhave reported; in general, the optimum seems to shift to higher values but to become less pronounced. The results of Osbaeck and J m s , and of Jelenic r t 01. (J25), indicate that it is more definitely upwards if the alkali content is high. In evaluating the effect of gypsum content on strength or other properties, it is necessary to ensure that other relevant factors are as far as possible kept constant. Thus, if cements with the same clinker but dityerent gypsum contents are ground to the same specific surface area, the clinker will be less finely ground the more gypsum is present, and this will tend to decrease the rate of strength development. Calcium sulphate additions probably affect both the quantities of products that are formed, especially at early ages, and the microstructure at all ages. Addition of CaSO, is reported to increase the rate of hydration, both of pure C3S and of the alite in cement (C39,B104,J25,J26,L36). This could explain an increase in strength a t early ages due to additions up to the optimum, but not a decrease with larger amounts. The fact that calcium sulphate additions affect the drying shrinkage also suggests that the micro-
236
Cement Chemistry
structure is modified. This could occur in various ways, e.g. by changes in the m~cromorphologyof the C-S-H or by incipient precipitation of ettringite throughout the gel. The latter effect, if it happened at an early age or to a limited extent, could augment strength through a decrease in porosity, but if it happened later o r to too large an extent could decrease strength through localized expansions. Attempts to formulate equations relating optimum gypsum content to composition and particle size distribution have proved of little practical value, and gypsum contents are normally decided on the basis of empirical strength tests. In practice, the molar ratios of total SO, to total AI,O, in modern cements range from 0.5 to 0.9, with an average of about 0.6 (K35). In order to adjust the supply of SO,2- to that required at different stages of the hydration process, it may be desirable to use mixtures of difrerent forms of calcium sulphate; for example, with cements high in aluminate phase, a mixture of anhydrite and hemihydrate, the latter produced by dehydrating gypsum during milling, has been recommended (L36).
7.6.3 Effects of alkalis Alkali cations normally occur in cements either as sulphates or in the major clinker phases. In either case, the balancing anion sooner or later enters a hydration product of low solubility, and an equivalent amount of O H - ion is released. Apart from syngenite precipitation, mentioned earlier, the effects of alkali cations on cement hydration are likely to be predominantly those of the O H - ion. Jawed and Skalny (524) reviewed these effects, both on the hydration process and on the properties of the hardened product. In general, early strengths are increased and late strengths decreased. Some other properties, such as drying shrinkage, are affected, and the optinlunl gypsum content is increased. This last effect must be taken into account when effects on strength are considered. Lerch (L38) attributed it to accelerated early reaction of the clinker phases. The results obtained by Osbaeck and J m s (08) in the study mentioned in the previous section arc probably typical. They show the effects of alka sulphates on early and late strengths and on optimum gypsum conten mentioned above. The effects on strength were diminished or absent gypsum contents above the optimum. QXDA showed that the increas early strengths were associated with increased reaction of the alite. alum nate and ferrite phases. This effect was quite marked; for example. at 1 da the reaction of the alite increased from 26% for a cement low in solub alkalis to 56% for an otherwise similar cement high in them. There were less definite indications of an association between the decreased strength at 28 days and decreased consumption of the clinker phases, and the authors were
,
Hydration of Portland Cement
237
uncertain whether the decrease in strength was due to this cause or to the formation of a less favourable microstructure. Bezjak c.f 01. (B98) similarly found that the fractions of alite reacting at ages were substantially increased in the presence of alkali sulphate. both in Portland cement and in mixtures of alite with C,A. After 10 days. which was the longest time studied, the fraction reacting was still enhanced in the presence of alkali sulphate in the case of a cement low in aluminate phase, but was slightly lowered in that of cement higher in aluminate. Applying a theory of the kinetics of hydration described in Section 7.7.2. they attributed the acceleration at early ages primarily to an increase in the permeability of the layer of hydration products surrounding the alite at a stage soon after the rate of reaction has become controlled by diffusion. Tang and Gartner (T34) studied a clinker low in alkalis and in SO,, but high in aluminate, which they blended with varying mixtures of calcium sulphate, calcium alkali sulphate and alkali sulphate phases. They, too. found that alkali sulphates increased the early strength; without exception. the I-day strengths obtained using blends of gypsum or hemihydrate with aphthitalite, syngenite o r calcium langbeinite were greater than those obtained at the same ratio of total SO, to A1,0, with the calcium sulphates alone. calcium langbeinite and syngenite being especially effective. The efficacy of calcium langbeinite as a set-controlling agent was earlier recognized by Moir (M7). At 28 days, in contrast to the findings of Osbaeck and J m s , there was little significant difference in strength between the cements with and without alkali sulphates, though those of the former tended to be lower. Jelenic cJf (11. (526) found that, for pastes of alite and C,A with varying amounts of gypsum and with or without alkali sulphate, high strengths at 28 days were correlated with low C,A consumption at 1 day. The late strengths were increased, not decreased, in the presence of alkali provided that the gypsum content was suitably adjusted. Tang and Gartner (T34) similarly found little correlation between the amount of aluminate phase consumed at 2 min and the compressive strength a t I day, but there was a negative correlation between this quantity and the strength at 28 days. They concluded that a reduced initial aluminate consumption tended to improve not only the workability of the fresh paste, but also the ultimate strength of the hardened material.
7.7 Kinetics and modelling of the hydration process 7.7.1
Kinetics: experimental data
The kinetics of cement hydration are concerned with the relations between
238
Cement Chemistry
the degree of hydration (a) and the age (0, and the factors that influence them. We shall describe curves of a against t as kinetic curves. From the practical standpoint, the kinetic curve controls the way in which the physical properties develop as curing proceeds. a may relate either to an individual clinker phase or to the cement as a whole. Because cement is a mixture of phases that react at diKerent rates, there are problems in determining, and even in defining, u for the whole cement; 'we shall define it as the weight fraction of the cement that has reacted, irrespective of phase. In principle, it may be obtained by summing the amounts of the individual phases that have reacted, which in turn can be determined by QXDA or in other ways. Because of the experimental dilficulty and questionable precision of this approach, less direct methods have often been used, based on the determination of such quantities as nonevaporable water, cumulative heat evolution (C38), or chemical shrinkage (K36). It has normally been assumed, with varying degrees of justification, that these quantities are proportional to a as defined above. Major influences on the kinetic curve of a cement include the phase con~positionof the clinker, the particle size distribution of the cement and the R H and temperature regimes during curing. Other influences include the w/c ratio, the content and distribution of admixtures, including gypsum, the reactivities of individual clinker phases and probably others. such as the microstructures of the clinker and of the cement particles. By definition, the kinetic curve of a cement is the weighted sum of the curves for its constituent phases as they occur in that cement. The reactivities of individual clinker phases were considered in Section 4.5 and some effects of particle size distribution, which is a particularly important variable, in Section 4.1.4. Although many data relating particle size distribution directly to strength exist, much less is known about its relation to degrees of reaction. Parrott and Killoh (P30) presented data indicating that the rate of hydration, as represented by that of heat evolution, was proportional to the specific surface area during the period of hydration in which the rate was controlled by diffusion. The rates of reaction of the clinker phases are greatly influenced by the RH of the atmosphere in which curing occurs. For a typical Portland cement paste of w/c ratio 0.59 cured at 20•‹Cand 100% RH, Patel P I rrl. (P28) found the fractions of the alite, belite, aluminate and ferrite phases hydrated after 90 days to be respectively 0.94, 0.85, l .OO and 0.5 1. If the RH was lowered to SO%, the corresponding values were 0.77,0.19,0.83 and 0.32. The hydration rate of the belite thus appears to be especially sensitive to RH. On the basis of earlier data from the literature, Parrott and Killoh (P30) concluded that the effect of R H on the hydration rate (daldt) of each of the phases could b represented by a factor ((RH - 0.55)/0.45}4.
Hydration of Portland Cement
239
Copeland and Kantro (C39), using QXDA, found that alite and belite in a portland cement hydrated more rapidly at w/c = 0.65 than at w/c = 0.35. Heat of hydration data (C38) indicated that the rate of hydration in Portland cement pastes is higher at w/c = 0.6 than at w/c = 0.4 at all ages from 1 day to 6.5 years. Taplin (T40) found that at low w/c ratios the rate was significantly reduced at later ages. Based on this last result, Parrott and Killoh (P30) considered that daldt was reduced by a factor ,f(w/c) which applied only at values of a greater than 1.333 x w/c:
In contrast to these results, Locher (L39), working with pure C,S, found that hydration was more rapid at a low w/c ratio. Temperature has a large effect, especially in the earlier stages of hydration; for example, from QXDA, Copeland and Kantro (C39) found that in a Portland cement paste of w/c = 0.57, the fraction of the alite hydrated at 2 days was 0.28 at 5•‹C.0.63 at 25•‹Cand 0.81 at 50•‹C. Apparent energies of activation calculated from such data were 41 kJ mol-' at u = 0.6 and 26 kJ mol-' at u = 0.7; for belite, a value of 56 kJ mol-' at a = 0.4 was obtained. The decrease in the apparent energy of activation in the case of alite was attributed to a gradual change in rate control from a chemical process to diffusion.
7.7.2 Interpretation of kinetic data The kinetics ofcement hydration are dominated by the effects associated with the particle size distribution of the starting material, and attempts to explain them in which this is ignored can lead to very misleading results (T41,B98,B105,K37,J27,K38,K39). Even laboratory-prepared samples with close distributions (e.g. 2-5 pm) (K20) are far from monodisperse from the kinetic standpoint. Two approaches to the resulting problems of interpretation will be considered. Bezjak, Jelenik and co-workers (B98,B105,J27) assumed that during the hydration of each particle, up to three processes were rate controlling at different stages. The first was nucleation and growth of a product. The second was a process occurring at a phase boundary. The third was a diffusion process. Each supplanted a predecessor when it became the slowest of the three, at a time that depended on the initial radius ( r ) of the particle; more than one rate-determining process could thus operate simultaneously. Given the rate equations and appropriate rate constants, it was possible to calculate the value of a at any desired time for a particle of given radius, and
240
Cement Chemistry
by summing the amounts of material that had hydrated for particles of all sizes, the value of a for the whole material. In the rate equations relating a to t for a single particle, the first did not involve r. In the second, a function of a was proportional to ( t - tO)/r,where t, is the time at which the process became rate determining. In the third, a function of a was proportional to (1 - t,)/r2. Kinetics represented by equations of the second and third of these types are described as linear and parabolic, respectively. I t was shown that the kinetic curves of a number of alite and cement pastes, some of which contained added alkali ~ulphate~, could be satisfactorily explained (B105). For the cements, diffision became virtually the sole rate-controlling process at values of a varying between about 30% and 60%. This appears to agree broadly with the evidence from apparent energies of activation noted in the previous section. Knudsen (K37-K39) assumed that the rate equation relating a to t for a single particle could be either linear or parabolic. Like Bezjak ct (11.. he also assumed that the total amount of material reacted was the sum of the amounts calculated for all the individual particles, and thus, given the rate equation for a single particle, the rate constant and the particle size distribution, obtained the value of a for the whole material. In contrast to Bezjak et d., he concluded that the results were highly insensitive to the precise form of the rate equation, and that all that could be learned about the latter from the kinetic curve was whether it was linear or parabolic, and the value of the rate constant. Using data from various sources, Knudsen concluded that, for the values of a > 0.15 to which his model was applicable, some cements followed linear kinetics for virtually the whole range of values up to a = 1 , and others parabolic kinetics. A change from linear to parabolic kinetics could be produced by adding CaCI,, and the possibility of a gradual change from linear to parabolic kinetics at high values of a was not excluded. It is not obvious how the conclusion that some cements show linear kinetics throughout can be reconciled with the evidence from apparent energies of activation. Knudsen's model led to the prediction that, if linear kinetics were followed. the age at which 50% of the cement has hydrated is proportional to the fineness constant r, (or x,) in the Rosin/Rammler distribution (equation 4.1); for parabolic kinetics, it predicted that this age is proportional to r02(K40). Evidence was presented in support of this conclusion for cements considered to follow linear kinetics. The theory did not predict any relation to the breadth of the particle size distribution, which is represented by the slope of the Rosin-Rammler curve. In neither of these approaches is it necessary to specify the rate-controlling step more precisely than is given above. The discussion on the hydration kinetics of C,S pastes (Section 5.6) is probably substantially applicable also
Hydration of Portland C e m e r ~ t 241
to cement pastes. One may speculate that in the theory of Bezjak c/ NI..the nucleation and growth of C-S-H determines the rate until the anhydrous phases cannot supply the ions sufficiently quickly. Dissolution of the anhydrous phases then becomes rate determining until the permeability of the layer of product has decreased to a level such that diffusion takes over. It would be highly desirable to examine the implications of both theories in the light of the more recent microstructural evidence (Section 7.4). I t would also be desirable to determine whether Knudsen's more pessinlistic conclusion regarding the information that can be extracted from the experimental data results from a more realistic appraisal of the precision of the data, or from the differences between his assumptions and those of Bezjak cr ul.
7.7.3
Mathematical modelling of the hydration process
This has the objective of quantifying knowledge of the process in a manner that permits the prediction of experimentally accessible properties from similarly accessible inputs, thus allowing the effects of changes in inputs or assumptions contained in the model to be examined. Necessary inputs comprise such data as cement composition, w/c ratio and curing conditions, and typical outputs include weight o r volume fractions of phases in a paste or concrete, content of non-evaporable water, cumulative heat evolution or physical properties such as strength, porosity or permeability defined in various ways. Output might also include quantitative description of microstructural features. The most comprehensive attempt to model hydration to date is due to Parrott (P30,P32.P33). In essence, it contains three steps, viz: ( I ) The degree of hydration of each clinker phase is calculated a t 56 ages. For each step, three different rate equations, each with assunled constants, were tested, and that giving the smallest increment of reaction was used. The rate equations were modified to allow for the effects of temperature, RH and w/c ratio as indicated in the previous section (P30). Two of the equations were similar to the first and third of those used by Bezjak and Jelenii (BIOS), the third being an empirical equation introduced to deal with the later stages of hydration. In so far as the clinker phases were treated separately, the simultaneous existence of more than one rate-determining step was assumed. Allowance was made for the effect of particle size distribution. (2) The phase composition of the paste by weight and by volume, and the non-evaporable water content, were calculated using procedures
242
Cement Chemistry
broadly similar to that described in Section 7.3.3. Porosities were then calculated by a method indicated in Section 8.3.1. The heat evolved was assumed to be the sum of the amounts given by each clinker phase, which in turn was assumed to be proportional to the fraction of that phase that had reacted. Extensions of the treatment permitted the effects of admixture of the cement with pfa to be considered. (3) The compressive strength, permeability to water and methanol exchange rate were calculated, using empirical equations relating them to the porosity; such relations are discussed in Chapter 8. Examples of predictions from the model were given, and the desirability of continued updating of the procedure in the light of new knowledge of the hydration process was indicated. Jennings and Johnson (528) described a mathematical model simulating the development of microstructure during C,S hydration. The anhydrous grains, treated as spheres having a realistic size distribution and randomly placed, were assumed to react forming layers of inner and outer hydrates, together with crystals of C H growing in the water-filled space, which were also assumed to be spherical. The model could clearly be extended to cement and refined in various ways, and the approach offers considerable promise as a means of exploring the microstructural consequences of differing assumptions regarding the particle size distribution and other characteristics of the unhydrated cement, the stoichiometry of the hydration reactions. and the densities and other properties of the hydration products. This type of model could perhaps be advantageously combined with that developed by Parrott. The representation of kinetics by appropriate equations is another aspect of modelling, some aspects of which have been reviewed by Brown et al. (B106). A more complete understanding of the kinetic laws governing the various stages of the hydration process, especially if it took into account the more recent microstructural evidence, would contribute to the power of models of the kinds described above.
Structure and properties of fresh and hardened Portland cement pastes 8.1
8.1. I
Fresh pastes
Workability
Workability is a general, descriptive term that indicates the ease with which a concrete can be mixed, transported, placed and compacted to give a uniform material. There is no single measure of it, but various empirical tests that provide information on particular aspects are widely used. The most important is the slump test, in which the material is moulded by lifting away a conical container in which it was placed. The subsidence of the resulting pat provides a measure of the ability of the material to flow under its own weight. Neither this, nor any of several other empirical tests that are in use, give results that are simply related to any fundamental rheological properties. Their main value is for quality control of a given concrete mix rather than for the comparison of different mixes. The workability of a concrete mix is by no means dependent only on the physical properties of the cement paste it contains, but an understanding of it requires one of those properties. For some specialist uses in which cement is used without an aggregate, the latter are directly relevant. The most important properties are concerned with rheology, and this section deals primarily with these properties in Portland cement pastes, free from admixtures, prior to setting. From the chemical standpoint, this period comprises that of initial reaction and induction period. From the practical standpoint. it includes those of mixing, placing and compaction.
8.1.2 Rheology The resistance of a fluid to flow may be considered in terms of the situation existing between two parallel planes, one of which is moving in its own plane relative to the other. It is assumed that flow is confined to the single direction thus defined, and that the velocity varies linearly with distance in the direction perpendicular to the planes. Liquids of simple and stable molecular structure generally obey the Newtonian law
244
Cement Chemistry
where t is the shear stress (Pa), q is the viscosity (Pa s) and y is the shear rate (s- I ) , equal to the velocity of one surface relative to the other divided by the distance between them. The resistance to flow at a given temperature and pressure is thus defined by a single constant, the viscosity. Suspensions show Newtonian behaviour only if there is no long-range structure. In varying degrees, many approximate in behaviour to the Bingham model, represented by the equation
where r, is the yield value and p is the plastic viscosity. No flow occurs until the shear stress exceeds r,. The relation between r and y may not be linear, and may show hysteresis. Fig. 8.1 shows some of the possibilities. A curve that is concave towards the shear rate axis (Fig. 8.1C) indicates breakdown of structure due to shear. Positive hysteresis (Fig. 8.1E) shows that the structure does not reform immediately on decreasing the shear rate. Some fluids show thixotropy, that is, the structure breaks down under shear stress and reforms slowly when this is removed. A second ascending curve will lie between the previous ascending and descending curves, whereas if the structure does not reform, it will coincide with the descending curve. Thickening under stress (Fig. 8.lD) and negative hysteresis (Fig. 8.1 F) are other possibilities. For various reasons, the curves obtained in practice can differ considerably in form from the idealized ones shown in Fig. 8.1, especially at low shear rates. The quantity r / j for a non-Newtonian fluid is called the apparent viscosity. Its value varies with j and, if hysteresis occurs, with the previous history of the sample. Rheological studies on cement pastes and concrete have been reviewed in a book (T42) and several shorter articles (H40,L40,S76). The results in the literature show wide variations, many of which reflect the large effects of seemingly minor differences in experimental technique. Since cement pastes d o not show Newtonian behaviour, methods giving only a single parameter are inadequate. Viscometers in which the material is studied in shear between a shallow cone and a plate, or between concentric cylinders. have generally been used. Tattersall (T43) found that pastes of w/c ratio 0.28-0.32 and age 4.5 min followed the Bingham model at low rates of shear, but that at higher rates the structure broke down irreversibly. Several other investigators have obtained similar results, but negative hysteresis has also been observed (e.g. Ref. R30), probably due to the use of hysteresis cycles of long duration, in which the structural breakdown due to shear is outweighed by the effects of
Structure and Properties of Portland Cement Pastes
245
continuing hydration (B107). The rheological behaviour is strongly affected by the method of mixing, since the initial structure is broken down if this produces sufficient shear. Other major factors affecting the plastic viscosity and yield value are the w/c ratio, particle size distribution, age, presence of admixtures (Chapter I I) and, to a lesser extent, cement composition. There are severe difficulties in interpreting hysteresis loops, and in studies of the variation in shear stress with time a constant speed of rotation has been found to give more certain information ( ~ 4 2 , ~ 4 0 ) .
Fig. 8.1 Idealized plots of shear rate ( j ) against shear stress (s) for fluids of various types. (A) Newtonian fluid. (B) Bingham fluid. (C) Shear thinning. (D) Shear thickening. (E) Positive hysteresis: 1, 2, 3A thixotropy; 1 , 2, 3B, rheodestruction. (F) Negative hysteresis with antithixotropy.
8.1.3
Models of fresh paste structure
Helmuth (H40) considered the cement particles in a paste to constitute a single floc, in which, however, there were regions of varying solid content. On shearing, the particles became more uniformly distributed, and the static floc structure was reformed on standing. Tattersall and Banfill (T42) considered that this model did not sufficiently explain the irreversible nature of the breakdown caused by shearing, and proposed that during the initial reaction aggregates of particles already in contact were coated with a continuous
246
Cement Chemistry
membrane of gel which, if destroyed by shearing, was replaced by separate coatings around each particle, which were less effective in binding the particles together. It is difficult to test such models directly, since studies on dilute suspensions are of doubtful relevance to pastes and neither light nor electron microscopy is readily applied, but Uchikawa et d. (U 14) described a method of specimen preparation for SEM examination. The paste was mixed for 3 min and, after various times, was placed on the surface of a metal sample holder already cooled in liquid N,.The sample holder was then further cooled by immersion in liquid N, and the specimen was examined at - 120'C. For a Portland cement paste without admixtures 5 min after mixing, flocs of small particles and of large particles with adhering small ones were observed, but there was no continuous floc structure. The average distance between flocs was about 3 pm. At 2 h, C-S-H and ettringite were observed on the grain surfaces and the flocs were more definite. At 6 h, the layers of hydration products were much thicker and a three-dimensional structure was formed through the linking of the larger grains through smaller ones. One would expect the final distribution of cement particles after mixing is complete to be reflected in that of the hardened paste, which can be more readily studied, but no studies on the relations between mixing conditions and hardened paste structure appear to have been reported. T o an extent that increases with the w/c ratio, fresh cement pastes exhibit the phenomenon of bleeding, i.e. settlement of the solid particles. The interparticle attractions are sufficiently strong that particles of all sizes settle at the same rate, typically about 2 pm s-'. Settlement also tends to increase the wlc ratio at the top and to decrease it at the bottom of the sample. It decreases with increased fineness o r increased early hydration rate of the cement. In a concrete, it can produce layers of water beneath aggregate particles o r reinforcing bars.
8.2 Hardened c e m e n t pastes: models of structure 8.2.1 The Powers-Brownyard model The rest of this chapter deals with hardened Portland cement paste (hcp), which has the properties of a rigid gel. It is a relatively rigid and strong solid of high porosity and internal surface area. Variations in the relative humidity (RH) of the surrounding atmosphere cause it to gain or lose water and also produced small but important changes in volume. It is sometimes called a xerogel, i.e. a gel from which the dispersion medium (pore solution) has been removed. Powers and Brownyard (P20) described the broad
Structure and Properties of Portland Cement Pastes
247
structure of the material by a model based largely on evidence from total and non-evaporable water contents and water vapour sorption isotherms. Powers later modified the model in minor respects (P34). In this model, hcp is assumed in the general case to comprise three components from the volumetric standpoint, viz. (a) unreacted cement, (b) hydration product and (c) capillary pores. Individual solid phases are not considered, whether in the cement or in its hydration products, which were collectively called 'cement gel'. This term may be found confusing, because it includes the CH, which forms relatively large crystals and cannot reasonably be considered part of a gel; we shall substitute the term 'hydration product'. The water present in the paste was categorized as evaporable or nonevaporable, the latter being defined in the later work (P34) as that retained on D-drying. Evaporable water, when present, was considered to reside partly in the capillary pores, and partly in so-called gel pores within the hydration product. This latter part was called gel water. The content of non-evaporable water, relative to that in a fully hydrated paste of the same cement, was used as a measure of the degree of hydration. Portland cement paste takes up additional water during wet curing, so that its total water content in a saturated, surface dry condition exceeds the initial w/c ratio. Evidence from water vapour sorption isotherms indicated that the properties of the hydration product that were treated by the model were substantially independent of w/c and degree of hydration, and only slightly dependent on the characteristics of the individual cement. The hydration product was thus considered to have a fixed content of non-evaporable water and a fixed volume fraction, around 0.28, of gel pores. The hydration product occupies more space than the cement from which it is formed, and the capillary pores were regarded as the remnants of the initially water-filled space. Their volume thus decreases, and that of the gel pores increases, as hydration proceeds. Evidence from water vapour sorption isotherms indicated that the hydration product was composed of solid units having a size of about 14 nm, with gel pores some 2 nm across (P34). The width of the capillary pores could not be determined from the available data, but they were considered to be generally much wider than the gel pores, though tending to become narrower as the water-filled space was used up, and thus in some regions indistinguishable from gel pores. We shall use the following symbols: w/c w/c* w,/c w,/c
initial waterlcement ratio, corrected for bleeding. critical value of w/c below which complete hydration cannot occur; typically about 0.38. = total waterlcement ratio in the saturated, surface dry state. = ratio of non-evaporable water to cement.
= =
248
~\,,/c
Cement Chemistry
ratio of evaporable water to cement in the saturated, surface dry state, equal to w,/c less w,/c. = specific volume of unhydrated cement; typically about 1 'c 3.17 x 10-4m3 k g w 1 . = mean specific volume of the gel water, taken here as 1 'g x m3 kg-'. 1.00 = mean specific volume of the non-evaporable water in m3 kg-'. Vn = density of pore solution in fresh paste, taken here as 1000 kg m-3. D, = mass of gel water per unit mass of cement in a fully hydrated paste; t?lg typically about 0.21. 111, = mass of non-evaporable water per unit mass of cement in a fully hydrated paste, typically about 0.23. u = mass fraction of cement hydrated. urnax = maximum value of a possible at w/c d w/c*. =
8.2.2 Minimum waterlcement ratio for complete hydration; chemical shrinkage Powers and Brownyard (P20) determined the non-evaporable and total water contents of pastes of many cements made up at different w/c ratios and cured for different lengths of time. In Fig. 8.2 the ratios of evaporable water to cement in saturated, mature pastes of a typical cement are plotted against those of total water to cement. The initial w/c ratios are shown by the scale at the top; up to about 0.06 kg of water per kg of cement is taken up during wet curing. The data show a distinct break at w/c = 0.38, and are fitted by the equations w,/c
=
w,/c - 0.227
for w/c > 0.38
(8.3)
w,/c
=
0.482 x w,/c
for w/c < 0.38
(8.4)
and
The value of w,/c at the break is 0.44. Similar results were obtained for other Portland cements. In the following interpretation of them, the small changes in total paste volume that occur during curing are ignored, and any air entrapped or deliberately introduced during mixing is regarded as being outside the system. Because the hydration product was considered to occupy more space than the cement from which it is formed, and since the total volume of the paste scarcely changes on hydration, complete hydration cannot occur if w/c is below a certain value. For the cement under discussion, this value is 0.38, and a mature paste of
Structure and Properties of Portland Cement Pastes
249
this w/c ratio consists entirely of hydration product. The hydration product contains, per kg of cement, 0.227 kg of non-evaporable water and. in the saturated condition, 0.21 I kg of gel water, making a total of 0.438 kg. The pastes with w/c < 0.38 consist of unreacted cement and hydration product. and thus have (w,/c)/(w,/c) equal to 0.21 1/0.438, or 0.482. The pastes with w/c > 0.38 consist of hydration product and capillary pores. which in the saturated condition are filled with water, and thus have w,/c equal to w,/c - w,/c.
0
0.2
wlc 0.4
0.6
0 .8
Fig, 8.2 Relations between the initial (w/c), total (w,/c) and evaporable (w,/c) water/cement ratios for saturated, mature pastes of a Portland cement. After Powers (P34).
Most investigators appear to have agreed with the observation that complete hydration cannot occur if w/c is below a value in the region of 0.38, but Rossler and Odler (R3 I ) reported that hydration was complete in a paste with wjc = 0.22, and further investigation may be required. On the PowersBrownyard theory, the value of the minimum depends on the volume ratio of hydration product to cement, which in turn depends on the gel porosity of the hydration product. It was assumed that this porosity was fixed by the inability of new particles to nucleate in pores smaller than a certain size and by that of existing particles to grow beyond a certain size. If a paste having w/c < 0.44 is cured under sealed conditions, there will be insufficient water to fill the gel pores completely, and the capillary pores will
250
Cement Chemistry
be empty. The effective R H will be low, and hydration will become very slow or stop, even though free space remains. This phenomenon is called selfdesiccation. Even at higher w/c ratios under sealed conditions, partial emptying of the capillary pores will retard hydration, and can produce irreversible changes in pore structure which reduce the final strength. I t is therefore important to provide an adequate supply of water during curing. If a paste is kept in contact with an excess of water during curing, the total volume of paste and water decreases. This effect is called chemical shrinkage, and may be used to follow the course of hydration. For this purpose, it has the merits that hydration need not be stopped and that the determination can readily be automated. Knudsen and Geiker (K36) described a device for automated determination on up to 30 samples.
8.2.3
Calculation of volumetric quantities
This section gives expressions and values for some important volumetric quantities. The data for the cement discussed in the previous section are adopted as typical. (I) The total volume of paste per unit mass of cement is V , +(w/c)/D,, where V, is the specific volume of the cement and D, the density of the pore solution in the fresh paste. Assuming a value of 3.17 x m3 kg-' for V, and one of I000 kgm-"or D,,. this gives + (w/c)/1000m3 kg-'. 3.17 x (2) The volume of hydration product per unit mass of cement reacted is obtained by considering the special case of a fully hydrated paste having the critical w/c ratio, w/c*, in which only hydration product is present; it is V, + (w/c*)/D,. For the general case, it is thus [V, + (w/c*)/D,] x a , where a is the degree of hydration. Inserting the values x a m 3 kg-'. of Vc, w/c* and D,, this gives 7.0 x (3) The volume of'unreucted cement per unit mass of cement is Vc x (1 x (1 - a ) m 3 kg-'. a), o r 3.17 x (4) The specijic volume of' the non-evaporuble water, V,. is obtained by taking the volume of hydration product less those of the cement from which it was formed and the gel water, and dividing by the mass of non-evaporable water. This gives [(w/c*)/D, - V;m,]/m,, or m3 kg-'. 0.74 x ( 5 ) The muximutn degree ofhydration, a,,,, for a paste with w/c < w/c*, may be calculated by equating the total volume of paste to the sum of the volumes of unreacted cement and hydration product; it is (w/c)/ (w/c*), o r (w/c)/0.38. This assumes that an unrestricted supply of
Structure and Properties of Portland Cement Pastes
251
water is available during curing. If no such water is available, the degree of hydration is limited by the amount of water present initially to a maximum value of (w/c)/(tn, + 171,). or (w/c)/0.44. (6) The ratio qf the volume of'hyclrution product to thut o f t h e cnnent from which it is formed may be calculated by dividing the former quantity by the volume of cement reacted per unit mass of cement. which is a x V,, giving 1 + [(w/c*)/(D, x V,)]. This gives a value of 2.20. (7) The porosity o f t h e hjviration product is given by dividing the volume of gel pores per kg of reacted cement by that of the hydration product. This gives V, x m,/[V, + (w/c*)/D,], o r 0.30. The value of 0.28 given by Powers and Brownyard was obtained assuming a different value for V,. It should be noted that this quantity is the volume of gel pores relative to that of all the hydration products, including the CH. Fig. 8.3 shows the mass and volume relationships for a fully hydrated, saturated paste of w/c ratio 0.5, calculated using the above expressions and values. Following Powers and Brownyard, the hydrated cement is treated from a purely volumetric standpoint as a composite of reacted cement, nonevaporable water and gel water. The specific volume of the non-evaporable water was assumed to be 0.73 x m3 k g - ' , and that of the capillary and gel water to be 1.00 x m3 kg-'. The results are approximate, for several reasons; for example, the pore solution is in reality not pure water, but an alkali hydroxide solution with a specific volume (for 0.3 M KOH) of m3 kg-'. about 0.986 x MASS ( k g )
VOLUME
(m3 x103)
APILLARY WATER
\ G E L WATER
ATER
WATER l Y ORATION PRODUCT REACTED CEMENT
Fig. 8.3 Diagram illustrating the Powers-Brownyard description of a fully hydrated and saturated Portland cement paste of initial waterlcement ratio 0.5. All quantities refer to 1 kg of cement.
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Cement Chemistry
8.2.4 Later models of hardened paste structure Brunauer and co-workers (B55,BIOS) considered that the gel particlcs of the Powers-Brownyard model consisted of either two or three layers of C-S-H, which could roll into fibres. D-drying caused irreversible loss of interlayer watcr, and the specific surface area could be calculated from water vapour sorption isotherms, which gave values in the region of 200111'g-~ for cement paste. Sorption isotherms using N, give lower values of the specific surface area; this was attributed to failure of this sorbate to enter all the pore spaces. Feldman and Sereda (F31,F32) regarded the gel as a three-dimensional assemblage of C-S-H layers, which tended to form subparallel groups a few layers thick and which enclosed pores of dimensions ranging from interlayer spaces upwards (Fig. 8.4). They considered that much of the gel watcr of the Powers-Brownyard model was interlayer water. Unlike Brunauer. they considered that the loss of interlayer water on D-drying was reversible; consequently, sorption isotherms using N,, not water, gave a true measure of the specific surface area. In a modification of this model, Dainion ct (11. (D30) considered that the gel consisted of particles having an internal structure similar to that shown in Fig. 8.4, together with pores having an equivalent radius of 1.6-100 nm. The smaller pores within the gel particles were 0.6-1.6 nm in equivalent radius, and pores of both types tended to have narrow entrances.
Fig. 8.4 Feldman-Sereda model of the structure of the C-S-H gel of Portland cement paste, showing C-S-H layers (lines), interlayer water molecules (crosses) and adsorbed water molecules (circles). After Ramachandran er d.(R32).
Structure a n d Properties of Portland Cement Pastes
253
In Wittmann's (W23,W24) 'Munich model', hcp is described as a xerogel, together with the crystalline constituents. The xerogel is considered to consist of separate particles, for which no particular internal structure is assumed. Emphasis is placed on the role of water molecules adsorbed on, or lying between, these particles. Below 50% RH, the particles are considered to be in contact, but at higher humidities, intervening water molecules exert a disjoining pressure which keeps them apart to a limited extent. The model was used primarily in the development of a theory of dimensional changes. The Powers-Brownyard model explains why complete hydration does not occur if w/c is below a certain value, and provides a partial explanation of the dependence of some important physical properties, especially compressive strength, on w/c and degree of hydration. No other satisfactory explanation of the minimum w/c needed for complete hydration appears to have been given, and the most important feature of the model is probably the conclusion that a given volume of cement can react only if an additional volume of free space approximately 1.2 times as great is available. The direct evidence that the relevant characteristics of the hydration product are independent of w/c and degree of hydration was, however, based largely on data from water vapour sorption isotherms for severely dried pastes. the interpretation of which has been strongly criticized (F13). Further tests of the correctness of the hypothesis are needed. The Feldman-Sereda model was based on the studies of sorption properties, porosities and relations between water content and physical properties. Alone among the proposed models, it is clearly compatible with the microstructural evidence and with the probable relationships between C-SH gel and crystalline compounds. It is incompatible with that of Brunauer, but not with the essential features of that of Powers and Brownyard in its original form if the nature of the gel porosity is reinterpreted. Calculations of bound water (Section 7.3.3) indicate that about a third of the gel porosity of the Powers-Brownyard model is interlayer space, the remainder being micro or fine meso porosity* of the kind shown in Fig. 8.4. However. as that figure illustrates, the boundary between interlayer space and micropores is ill defined. The models differ in whether, to what extent, or on what scale the gel is regarded as being composed of separate particles. The microstructural evidence suggests that the situation differs with the region considered. the inner or late product being an essentially continuous, though porous, assemblage of layers, whereas the outer product is composed of similar material but is partly or wholly particulate on a 100-nm or micrometre scale. Powers (P34) discussed his model in relation to the meagre evidence on *The International Union of Pure and Applied Chemistry has adopted the follow~ngdefinitions of pores by width: rnicropores, < 2 nrn; rnesopores, 2-50 nm; rnacropores. > 50 nm.
254
Cement Chemistrv
microstructure then available. He recognized that the inner and outer products might differ in microstructure and that the porosities defined in the model could be means, but considered that the hydration product would tend towards a uniform porosity. In recent years, much progress has been made both in the study of microstructure by electron microscopy and i n that of pore structure by other methods, but relatively little has been done to relate the two approaches. Further understanding of the structure of cement paste requires a more complete synthesis of the existing models. the results of investigations on microstructure and on pore structure, and information of the structure of C-S-H and other hydration products at the crystal chemical level.
8.3 Pore structure 8.3.1
Porosities obtained by calculation
In the Powers-Brownyard model, the volume of capillary pores per unlt mass of cement is obtained by subtracting the volumes of hydrat~onproduct and of unhydrated cement from the total volume; using the symbols defined in Section 8.2.1, this gives [(w/c) - a(w/c*)]/D,. The volume of gel pores per unit mass of cement is equal to rn, x V, x a . Expressed as fractions of the total volume, the capillary porosity is [w/c - a(w/c*)]/[(D, x V,) + wjc] and the gel porosity is m,V, x a/[V, (w/c)/D,]. In all the above expressions, the maximum possible degree of hydration, a,,,,, equal to (\v,c)' (w/c*), must be substituted for a if w/c < w/c*.
+
Table 8.1
Calculated porosities, based on the Powers-Brownyard niodcl
Ratio
Fraction of ccmcnt hydrated (a)
Capillary porosity
Gel porosity
Total v, atcr porosity
0.3 0.3
0.00 0.79
0.49 0.00
0.00 0.27
0.49 0.27
uc
Structure and Properties of Portland Cement Pastes
255
Powers and Brownyard defined the total porosity as the sum of the capillary and gel porosities. Because the term 'total porosity' has been used by some later workers in other senses, we shall refer to such values as 'total water porosities'. As hydration proceeds, the capillary and total water porosities decrease and the gel porosity increases. Table 8.1 gives some values of the three types of porosity obtained from the above forniulae. using the data given in Section 8.2.2 as typical. ~
-
I
I
1
I
P
WIC Fig. 8.5 Relations between porosities (volume percentages) and waterjcement ratio for mature Portland cement pastes. The experimental data are for pastes at least 8 months old, and the calculated curves relate to a typical cement aged 18 months. Open symbols: total water porosities. Filled or half-filled symbols: mercury porosities. Curve A: total water porosity. Curve B: free water porosity. Curve C: capillary (S77); A (F33); V (M68); (S78): A porosity. References to data: 0 (P20); (F34); 0 (019); V (M68); 0 (D31); @ (H41). In the last two cases, porosities by volume were estimated from data referred in the original sources to masses of dried paste, assuming the latter to have contained 0.23 kg of water per kg of cement having m3 kg-'. a specific volume of 3.17 x
.
In Section 7.3.3, a method was described for calculating the quantitative phase composition of a cement paste by weight and by volume for various drying conditions. Fig. 8.5 includes porosities thus calculated for 18-n~onth-
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Cement Chemistry
old pastes of a typical Portland cement at varying w/c ratios and corresponding to (A) the D-dried state, (B) a state that includes all bound water, including that present in interlayer spaces, but no water in larger pores, and (C) capillary porosities, as defined by Powers and Brownyard. Conditions (B) and (C) are possibly realized approximately by taking material not previously exposed to an atmosphere below saturation and equilibrating it at I I % RH and about 90% RH, respectively (F14.Y5). The porosities defined by condition (B) will be called free water porosities. Parrott and co-workers (P30,P32,P35.P33) described a more sophisticated method for modelling the hydration process. The fraction of the tot,
8.3.2 Experimental methods: general points The problems of defining and determining densities of finely porous solids, indicated in Section 5.3.3, apply to hcp. A determination of the density of such a material is also one of porosity, since both properties are related to the solid volume; the porosity per unit volume of material is equal to I [tn,/(D, x V)], where m, and D, are, respectively, the mass and density of the solid and V is the total volume. The density and porosity determined by any method entailing contact with a fluid can vary with the extent to which the solid has been dried, how it has been dried and the fluid employed. Fluids may differ in their abilities to penetrate the pore system, and the pore structure may be altered both during drying and by the action of the fluid subsequently introduced. These comments apply also to studies of pore size distribution or specific surface area, which have been widely studied using sorption isotherms or, in the former case, mercury intrusion porosimetry (MIP). Gregg and Sing
Structure and Pro~ertiesof Portland Cement Pastes
257
((364) have described these methods. In both cases, the material must first be dried to water contents lower than those likely to be encountcred in the normal environment of the material, and the effects can be serious and difficult to allow for. The amount of water considered to form part of the solid obviously affects the definition of porosity, but the extent and manner of drying also affect the pore size distribution. The most widely ~ ~ s drying ed procedures have been desorption at ordinary temperatures, including D-drying, or at elevated temperatures, and solvent replacement. Freeze drying has also been used (K41). In solvent replucement, the specimen, normally in a saturated state, is immersed in an organic liquid, such as methanol. The course of replacement can be followed from the weight change; it has been claimed that all the pore water can be replaced (P36,D32). The organic liquid is then removed. Because of the lower surtitce tension, one might expect that less damage would occur as the liquid evaporates, and several investigators have concluded that this is the case (P36,H41,D32). In contrast, Beaudoin (B109,B45) concluded that straight chain aliphatic alcohols seriously affect the pore structure, partly through reaction with CH. Methanol and other organic liquids are retained strongly by cement or C,S pastes. with resulting effects on the T G and DTA curves (Section 5.1.2). It does not necessarily follow that the pore structure is alrected; this applies especially to the coarser part of the distribution, which is probably the more important for many of the physical properties. Porosities have sometimes been reported as percentages by volume of the paste and sometimes as volumes per unit weight of dried paste. The first is the more meaningful, especially as in the latter case the water content of the material has not always been given. A similar comment applies to specific surface areas (L41).
8.3.3
Determination of porosities by pyknometry
Total water porosities are obtained experimentally from the loss in weight when a saturated paste is D-dried o r subjected to some procedure regarded 21s equivalent, such as heating to constant weight at 105•‹C.Reabsorption of water by materials thus dried gives identical results (032). The procedures should be carried out under C0,-free conditions. A value for the specific volume of the evaporable water must be assumed; this has usually been 1.00 x m3 kg-'. Fig. 8.5 includes typical values thus obtained. Powers and Brownyard (P20) found that porosities of D-dried pastes obtained using helium as the pyknometric fluid were lower than those obtained using water. Feldman (F33) confirmed this and found that a lower value was also obtained using methanol; if a D-dried sample was exposed to
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Cement Chemistrv
helium, the latter flowed in quickly at first, and then more slowly. The porosities calculated from the initial, rapidly accepted volumes of helium agreed with those obtained with methanol. If the material was dried only by equilibration at 11% R H , the porosities obtained with helium, methanol or saturated aqueous CH were all similar to those obtained for D-dried pastes using methanol. For D-dried pastes, the porosities obtained using helium were 23.3%, 34.5%, 42.1 YOand 53.4% for w/c ratios of 0.4, 0.5, 0.6 and 0.8, respectively. The corresponding values obtained with water were 37.8% 44.8%, 51.0% and 58.7%; these are included in Fig. 8.5. Day and Marsh (D32) confirmed that porosities obtained by admitting methanol or propan2-01 to an oven-dried paste were substantially lower than total water porosities; however, if the undried paste was treated with the organic liquid, the volume exchanged was closely similar to the total water porosity. Feldman (F33) explained his results in terms of the Feldman-Sereda model as follows. If the paste is D-dried, water is lost from the interlayer spaces. I f the sample is immersed in water, this water is reabsorbed. and the total water porosity therefore includes the volume of the interlayer space. Methanol does not penetrate into this space, and helium does so only slowly, so that lower porosities are obtained. If the sample is equilibrated at I I 0 / 0 RH. the interlayer spaces are largely filled, and the lower value is obtained irrespective of the fluid used. The helium porosities reported by Feldman were near to the calculated free water porosities.
8.3.4
Sorption isotherms; specific surface areas
Sorption isotherms, using water, N2 or other sorbates, have been widely used to study the pore structure of hcp. The results obtained by different investigators show considerable variations, which arise in part from differences in the conditions of preliminary drying. Desorption of water from a saturated paste at ordinary temperature is a slow process. Fig. 8.6 gives results for a fully hydrated C,S paste of w/c 0.5, in which each point represents the results of a separate experiment on a slab I mm thick ((365). Loss of water is especially slow at 30-80% RH. Parrott et 01. (P37,P36) similarly found that in the initial desorption, equilibrium was reached within a few days at RH < 3O0/0, but that at RH values of 4O-80%, many weeks were needed; there were indications that larger pores emptied through smaller ones. Drying at these humidities led to a coarsening of the pore structure, which was indicated by a marked reduction in the specific surface area accessible to N,,from over 100 m2 g-' at 90% RH to around 30 m2 g - ' at 10% RH and was also associated with irreversible shrinkage (P38). On resaturation, the higher surface area was regained, but the pore size distribution did not completely revert to that originally found.
Structure and Properties of Portland Cement Pastes I
1
I
259
I
DRYING TIME (DAYS)
RELATIVE HUMIDITY
(O/o)
Fig. 8.6 Desorption curvcs for an initially saturated C,S paste of w/c ratio 0.5. hydrated for 5.8 years at 25•‹C. and in the form of slab samples I rnm thick. Each point represents the result o f a separate experiment in which the sample was exposed to an atmosphere of given relative humidity for the drying time shown. After Grudemo (G65).
Water sorption isotherms for hcp show marked hysteresis. Powers and Brownyard (P20) found that, while it was difficult to obtain reproducible desorption curves, the low-pressure part of the water vapour resorption curve varied little with w/c ratio, between different Portland cements, or, if allowance was made for the contents of unreacted cement, with the degree of hydration. This was their main direct evidence for the conclusion (Section 8.2.1) that the properties of the hydration product considered in their model \\ere essentially independent of these variables. However, the water sorption iostherms obtained by different investigators have varied considerably (e.g. Refs P20 and S79), and it is not clear to what extent the above conclusion \\,auld stand had different desorption conditions been used. In principle, isotherms at low partial pressures of the sorbate may be used to determine specific surface areas by the Brunauer-Emmett-Teller (BET) method (G64). In this method, it is assumed that molecules of the sorbate are adsorbed on surfaces that can include the walls of pores, provided that the distance between molecules on opposing walls is large compared with molecular dimensions. From a plot derived from the isotherm, and given the effective cross-sectional area of the sorbate molecule, the specific surface area of the sorbent and the net heat of adsorption are obtained. Using water as sorbate, specific surface areas of about 200 mZ per g of D-dried paste have typically been obtained for mature cement pastes of normal w/c ratios
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Cement Chemistry
(P20.H42); however. much of the watcr has been shown to enter the vacated interlayer spaces, thus rendering the BET equation at best only partially applicable ( F 3 1 ). Using N2 or other non-polar sorbates, widely varying and usually much lower values have been obtained. Lawrence (L41). reviewing earlier studies, noted that for well-cured pastes, the values ranged from ~ ~ n d e10r to nearly 150 m2 g - ' , and even higher values, up to 249 m2 g - ' . have been reported (L42). In contrast to values for fully hydrated pastes obtained using water, the N, areas tend strongly to increase with w/c ratio, but are also much affected by the conditions under which the sample has been dried. I t would appear that the value obtained using water. whatever its precise significance, is dominated by the structure at o r near the individual layer level and 1s thus relatively insensitive to coarser features of the structure. With a possible reservation regarding material dried by solvent replacement. N 2 cannot penetrate the interlayer spaces and can therefore give information about structural features at a somewhat coarser level.
8.3.5
Pore size distributions
The hysteresis shown by sorption isotherms of hcp, whether using water, N,, or other sorbates, indicates the presence of mesopores (G64). At partial pressures above those to which the BET theory applies, capillar! condensation occurs. If a liquid forms a meniscus in a cylindrical pore. its vapour pressure is lowered to an extent that increases with decreasing pore radius. Using an equation due to Kelvin, or other procedures ultimately derived from it, the isotherm may be used to estimate pore size distributions in the approximate range of 1-30 nm and may give indications regarding the shapes of the pores. It may be necessary to allow for the fact that the meniscus forms in a pore narrowed by an adsorbed layer of sorbate. From an early study of this type on D-dried cement pastes. Mikhail ~t (11. (M69) showed that the size distribution of the pores available to N2 was probably unimodal and that with increase in w/c from 0.35 to 0.7 i t shifted to higher values in both the low and the high parts of the distribution. It is difficult to reconcile this conclusion with the unvarying pore size distribution within the gel implied by the model of Powers and Brownyard. Lawrence and co-workers (L41,L43,L44) concluded from sorption data using N,and butane that the microstructure partly collapses during normal drying. This agrees with Parrott's (P37,P36) conclusion, noted earlier. Pastes that had been rapidly dried were likely to be closest in structure to undried pastes; the sorption results indicated that their structures were dominated by platey particles or lamellae that formed slit-shaped mesopores and micro-
Structure and Properties of Portland Cement Pastes
261
pores. Slow drying produced small pores more nearly cylindrical or spherical. Methanol treatment at least partially prevented the microstruct~~ral collapse, but did not always produce an increase in the recorded surface area, though it always caused a change in the shape of the isotherm; this was possibly due to chemisorption of the methanol. Using N, sorption on samples solvent-dried by methanol exchange. Hansen and Almudaiheem (H41) found the volumes of pores wtth diameters smaller than 4 nm and accessible to N, to be 0.052 nll, 0.035 111l and 0.038 ml per g of dry paste for mature pastes of w/c ratio 0.4, 0.6 and 0.75 respectively. These results correspond to volume porosities of about 9%. 5% and 4%, respectively. Patel et ul. (P28) determined the volumes of pores smaller than 4 11111 or 37 nm in methanol-exchanged samples, using a methanol sorption method in which porous glass granules of known and approximately monodisperse pore size were used as a reference standard. Using also the total water porosity, the volumes in the ranges larger than 4nm and 37 nrn were also obtained. For a cement paste of w/c 0.59 cured for 90 days at 100% R H . this gave values of approximately 47% for the total water porosity, 17% for pores above 4 nm, and 12% for pores above 37 nm. For a paste of wlc ratio 0.71 cured in the same way, the corresponding values were approxinlately 57%, 36% and 18%. These results indicate volumes of pores below 4 nm considerably greater than those reported by Hansen and Alniudaiheem. Parrott et al. (Pl9) used butane sorption to determine the volume of pores smaller than 50 nm. For a 28-day-old alite paste of w/s 0.59. a volunle porosity of 27% was obtained. 8.3.6
Mercury intrusion porosimetry (MIP)
This method is based on the fact that a liquid that does not wet a porous solid will enter its pores only under pressure. If the pores are assumed to be cylindrical, the pressure (p) needed to force the liquid into them is g i \ m by the Washburn equation
where y is the surface energy of the liquid, 8 is the contact angle, and t i is the pore diameter. For Hg, y = 0.483 N m-'. For y, values between 1 1 7 and 140" have been variously assumed. In critical review, Good ((366) concluded that, while a case for 180" could be made, it was probably best to assume a value of 130" in the absence of direct experimental data. The maximum pressure employed is typically around 400 MPa, which allows pores of nominal diameter down to about 3.5 nm to be intruded.
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Cement Chemistry
Typical results (Fig. 8.7) show that the distribution moves to smaller values as hydration proceeds. The observed porosity is mainly in the 3 nm to I pm range for young pastes, and in the 3-100 nm range for mature pastes. For mature pastes of low w/c ratio, which according to the PowersBrownyard theory consist entirely of hydration product, nearly all the porosity is below 50 nm (S77). We shall refer to the porosities obtained using mercury at the maximum pressures employed as mercury porosities. Typical values for mature pastes (Fig. 8.5) are somewhat lower than the calculated free water porosities.
(urn)+
EQUIVALENT PORE DIAMETER 5 2 ff 500 200 50 20
-
DAY DAYS DAYS YEARS
-
0
1 3 28 1.5
1 (nm)
-
0.5 5 ABSOLUTE PRESSURE
50
5 00
( M Pa)
Fig. 8.7 Mercury porosimctry curvcs for a Portland ccmcnt pastc (w/c ratio 0.47) at various agcs. Aftcr Fcldman (F34).
The earlier literature contains several references to 'missing' porosity, meaning the difference between mercury porosities and total water porosities. The difference clearly arises mainly from the failure of mercury to enter vacated interlayer space but has also been attributed to the encapsulation of gel by CH (D33) or to the presence of large ( > 15 pm), closed pores that are not entered (A 15). MIP results d o not show the sharp distinction in size between capillary and gel pores postulated by Powers and Brownyard (P20). Mikhail c2r a/. (M69) earlier came to the same conclusion from results obtained by N, sorption. With increasing age, the volume of pores smaller than about 20 nm increases and that of pores larger than about 200 nm decreases, but there is no sign of two separate distributions (Fig. 8.7). The single distribution
Structure and Pro~ertiesof Portland Cement Pastes
263
indicated by MIP corresponds to the capillary porosity and part of the gel porosity. For pure Portland cement pastes, the pore size distributions obtained using MIP are reported to be coarser for oven-dried materials than for ones dried by a solvent replacement method (M70), though D-drying is reported to give similar results to drying at 11% RH (BI 10). The results for pure Portland cement pastes that have been obtained using MIP have been reasonably self-consistent, but several ditEculties arise when they are compared with those given by other methods. SEM evidence (Section 7.4.3) shows the presence in significant quantity of hollow-shell grains considerably larger than I pm in size, which could only be entered through much smaller pores, but MIP of mature pastes shows scarcely any porosity larger than I ptn (Fig. 8.7). The closeness of the mercury porosities intruded at maximum pressure to the calculated free water porosities implies that there is little porosity in the 0.5-3.5 nm range. This is difficult to reconcile with evidence from N, or methanol sorption, which indicates the existence of considerable porosity in this range. The volumes of pores wider than 4 nm found using methanol sorption (P35,P28) are much smaller than those indicated by MIP. On the other hand, Hansen (H43) found that, for solvent-exchanged pastes, the sum of the volumes of pores below and above 4 nm. obtained using N, sorption and MIP, respectively, agreed with the total water porosities. Studies on other materials show that M I P determines the width distribution of pore entrances and not of the pores themselves (D34). The intrusion of mercury may also coarsen the pore structure; this need only imply that, at the higher pressures employed, some of the foils of the gel are displaced so that some pores are widened and entered while neighbouring ones are closed up. The combined result of these processes would be to produce a distribution narrower than that existing before the intrusion began. and a value for the porosity at maximum pressure that corresponded to a minimum pore width before intrusion of less than 3.5 nm. Experiments in which the mercury was removed and subsequently reintruded have indicated that the structure is usually not altered in the case of Portland cement pastes, though it is in that of pastes of composite cements (F35,D32). but cannot show whether an irreversible change occurred during the first intrusion.
8.3.7
Other methods
Parrott pr ul. (P19) impregnated alite pastes with a resin, and prepared polished surfaces which were etched with acid to dissolve the cement paste before examination by SEM. Correlations were found between the areas of
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Cement Chemistry
pores thus determined and the volumes of pores greater than 50 nm obtained by the difference between total water porosities and the contents of small pores found using butane sorption. Subsequently, backscattered electron imaging of unetched polished surfaces, with image analysis, has provided a more direct approach, which also gave results correlating with ones obtained by a sorption method (S38,S80). Scrivener (S80) described the considerable stereological problems inherent in obtaining three-dimensional information about such features as pore connectivity from two-dimensional sections and methods for dealing with them. The remaining methods to be described can be applied to undried material. Small angle X-ray scattering gives information on the areas of surfaces between regions of markedly different electron density. For saturated, mature cement pastes of w/c 0.4-0.6, Winslow and Diamond (W25,W26) obtained values of 700-800 m Z per g ignited weight, decreasing to around 200 m 2 g - ' on D-drying. The decrease with loss of water was reversible if the specimen was not heated. Correction for C H and unreacted clinker phases would increase the values for the saturated pastes to around 1 0 0 0 m 2 g - ' . The authors were uncertain whether the method would be sensitive to an interlayer of water molecules, but the value obtained is about what would be expected for layers of C-S-H thus separated. The decrease on drying could be attributed to the coming together of layers. and its reversibility agrees with the hypothesis that the loss of interlayer water on Ddrying is reversible. Small angle neutron scattering may be used in a somewhat similar way. It is especially sensitive to concentrations of hydrogen atoms. A study of the 325 nm range by this method showed a bimodal distribution of pores peaking at approximately 5 nm and 10 nm, but accounting for less than 2% of the total porosity (A16,P39). The pores were approximately spherical. and on heating the material a t 105", partial collapse of the pore structure was observed, with loss of the 10 nm peak. Proton N M R by pulsed techniques to permit the study of relaxation effects provides information on the distribution of protons, and thus of water molecules, between different environments. An early study (S81) indicated that, at RH < 70%, the evaporable water in hcp was in an environment similar to that of the interlayer water in clay minerals or in certain crystalline hydrates. Subsequent work (Bl l l,M71,M72,S82) has given indications of the distributions of both chemically bound and unbound water among a number of environments. The method, which can be applied most effectively only to materials very low in paramagnetic atoms (including iron), has so far been used primarily to follow the course of hydration, but it would appear that it could also provide much information on pore structure.
Structure and Properties of Portland Cement Pastes
265
Low-temperature calorimetry (S83,B112) has been used to study coarse porosity. The method is based on the fact that water in pores freezes at a lower temperature than water in bulk. The ice forms through the advance of a front, analogous to the intrusion of mercury or the desorption of water. Hysteresis effects indicated the existence of necks in the pores. and the occurrence of up to three distinct peaks on curves of apparent heat capacity against temperature was interpreted as indicating maxima in the pore size distribution. Coarsening of the pore structure on drying was confirmed.
8.4 Strength 8.4.1
Empirical relations between compressive strength and porosity
The physical properties of concrete are only partly determined by those of the cement paste that it contains. We shall consider these properties only for hcp and to the extent that chemical or microstructural studies have contributed to an understanding of them. Other aspects are treated elsewhere (e.g. Ref. M73). Factors determining the compressive strength of a cement paste include (i) the characteristics of the cement, such as clinker composition and microstructure, gypsum content and particle size distribution, (ii) the w/c ratio and the contents of air and of any admixtures present in the mix, (iii) the mixing conditions, (iv) the curing conditions, especially temperature and RH. ( v ) the age and (vi) the manner of testing, including the water content of the specimen. The effects of temperature and admixtures are considered mainly in Chapter 1 1 and some additional factors relevant to mortar or concrete are considered in Section 12.1. In various ways, the factors listed above determine the degrees of hydration of the clinker phases and the phase composition and microstructure of the hardened paste, which in turn determine its physical properties, including strength. Many empirical relations between compressive strength and one or more of these variables have been proposed. Thus, Feret's law (1892) states that the strength is proportional to [c/(c + w + a)I2, where c, w and u are the volumes of cement, water and air, respectively, and various authors have reported equations based on regression analyses relating strength to cement composition and other variables (e.g. A17,A18). The generality of such equations appears to be limited, probably because of the difficulty of taking into account all the relevant parameters. The porosity of the hardened paste, appropriately defined, is strongly correlated with strength. As was seen in Section 8.3.1, it depends primarily on the degree of hydration and the w/c ratio. In this discussion. the following symbols will be used:
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Cement Chemistry
= compressive strength = hypothetical maximum compressive strength attainable o, = porosity, which has usually been taken to mean total water porosity p = hypothetical porosity at or above which the strength is zero p, A. B. C, D and E are constants
o
Powers (P34) found that cement pastes of various degrees of hydration and w/c ratio conformed to the relation
where X is a quantity called the gel/space ratio, and equal to the volume of hydration product divided by that of hydration product plus capillary porosity, both quantities being defined as in Section 8.2.1. The value of A was about 3.0 if non-evaporable water was used as a measure of the degree of hydration, and typical values for o, were 90-130 MPa. This equation breaks down for mature pastes of low w/c ratio. because it implies that the strength does not then depend on the w/c ratio. In realit). strength increases with decreasing w/c ratio, even though some of the cement does not hydrate. Several direct relationships between porosity and strength. originally found to hold for other materials, have been applied to cement pastes, I+: o = 0, (I -p)H(BI13) o
=
o, exp ( - Cp) (R33)
o
=
0
D In (p,/p) (S84)
= 0, ( I - Ep) (H44)
Equation 8.7 is similar to that of Powers, but unreacted cement is considered equivalent to hydration product. Some of these equations break down at zero or high porosities, but for a wide range of intermediate porosities. assuming suitable values of the constants, any of them can give a reasonable fit to a given set of data. Rossler and Odler ( R 3 1) concluded that equation 8.10 was the most satisfactory. A relation identical to that of this equation, using capillary porosity, is implicit in a figure represented earlier by Verbeck and Helmuth (V5).
8.4.2
Relations between strength and microstructure or pore size distribution
There are several indications that the compressive strength does not depend solely on porosity. In general, procedures that accelerate early reaction and
Structure and Properties of Portland Cement Pastes
267
thus increase early strength, such as increase in temperature or addition of certain admixtures, tend to decrease later strength. I t is unlikely that they significantly alter the porosity of the mature paste, but there is evidence that they affect the microstructure. If one extends consideration to a wider range of materials than normally cured Portland cement pastes, the relation between strength and porosity is markedly dependent on the broad characteristics of the microstructure. Jambor (529) concluded that both the volume and the specific binding capacities of hydration products must be considered.
POROSITY Fig. 8.8 Relations between compressive strength (log scale) and porosity: datx from various sources for Portland cement pastes cured at ordinary temperatures (open circles), hot pressed Portland cement paste (filled circle), autoclaved pastes oT Portland cement, sometimes with addcd sulphur (filled squares) and autocla\ed pastes of Portland cement with 50% (open squares) or 30% (cross) of addcd pfa. After Feldman and Beaudoin (F36).
Feldman and Beaudoin (F36) showed that, while the strengths of Portland cement pastes could be fitted to a curve in accordance with equation 8.8. materials of a broadly similar type but composed largely of denser, crystalline particles were, for a given porosity, stronger a t low porosities and weaker at high porosities (Fig. 8.8, curves AB and CD). These materials comprised hot pressed cement pastes (Section 1 1.9.1) and autoclaved cement pastes to which no silica had been added (Section 11.7.2). Feldman and Beaudoin considered that, while porosity was the major factor controlling strength, the morphology and density of the particles were also important. At high porosities, the better bonding properties of ill-crystallized material
268
Cement Chemistry
augmented the strength, but at low porosities, the greater intrinsic strength of the dense, crystalline particles was more important. In agreement with Bozhenov ct 01. (BI 14), they concluded that the highest strengths for a given porosity could be obtained from an appropriate blend of the two types of particles. Such a blend could be realized in autoclaved cement-silica materials (curve EF). The size and shape distribution of the solid particles in a cement paste is related. though in a complex way, to that of the pores, and other workers have attempted to relate strength to pore size distribution. From studies on autoclaved materials, M indess (M74) concluded that, for a given porosity, the strength increased with the proportion of fine pores. Jambor (529) similarly found relations between strength and average pore size. The weaker materials, with relatively coarse pores, were ones containing dense, crystalline phases, and broadly resembled those represented by the highporosity end of curve CD in Fig. 8.8. Odler and Rossler (020) concluded from a study of cement pastes cured for various con~binationsof w;c, temperature and time that while the main factor influencing strength was porosity, pores with a radius below 10 nm were of negligible importance. This conclusion, which was based on results of MIP, was supported by the observation that the specific surface area, which depends on the content of very small pores, had very little effect on the strength. For a given porosity. a given volume of hydration products contributed more to the strength than the same volume of unreacted cement, but this effect was distinctly less than that of porosity. These results suggest that, in attempts to relate strength to porosity. the total water porosity should not be used, the capillary or free water porosity or the volume of pores above a certain size being more appropriate. Parrott and Killoh (P30), in relation to their modelling of properties. similarly considered the volume, size and continuity of the larger pores to be the relevant quantities.
8.4.3
Mechanisms of failure
Strength cannot be explained by relating it empirically to porosity or pore size distribution; it is necessary to know what holds the material together and what happens when it fails. Many types of interatomic force exist in cement paste, and any distinction between chemical and physical forces is without obvious meaning. It has sometimes been assumed that van der Waals forces are important, but if this term is used in the restricted sense of London forces, this is unlikely. Although such forces exist between any atom and its near neighbours, in so strongly polar a material as hcp they are
Structure and Properties of Portland Cement Pastes
269
probably almost everywhere swamped by much stronger forces, comprising ionic-covalent bonds, ion-dipole attractions (e.g. between C a 2 + ions and H,O molecules) and attractions between permanent dipoles, including hydrogen bonds. London forces are, however, probably the dominant attractive forces between the layers within a C H crystal. Cohesion has often been attributed to the interlocking of fibrous or acicular particles. This could be important in the more porous parts of the material, but in the material as a whole, attractive forces between those parts of adjacent layers of C-S-H or other phases that are in contact are probably more important, both within particles and, in so far as the material is particulate, between them. The attractive forces could be direct. of the types mentioned above, or indirect, through interposed H,O molecules forming ion-dipole attractions and hydrogen bonds. Even for D-dried material. analogies with crystalline tobermorite and jennite indicate that much interlayer water is still present (Section 5.4). Various authors have discussed the application of fracture mechanics to cement paste (J29,A19,B115,P40,E4). As in other brittle materials. failure occurs through the initiation and spread of cracks, which originate in places where the local stress is high. Hardened cement pastes are about ten times as strong in uniaxial compression as in tension. It is probable that the ultimate mechanisms of failure are essentially similar, but that the cracks propagate more readily under tension. The relations between microstructure and strength must thus be sought in the features that give rise to high local stresses and in those that favour, or, conversely, that arrest the spread of cracks. This is consistent with the conclusion that porosity is a major factor but not the only one; Beaudoin and Feldman (BI 15) discussed some others. in addition to those noted above. The strength, however measured, is less for a saturated paste than for a dry one. In principle, this is readily explained, as the entry of water molecules between particles could weaken the attractive forces between them and thereby favour the initiation or spread of cracks. A disjoining pressure is created (Section 8.5.2).
8.5 8.5.1
Deformation
Modulus of elasticity
Curves of uniaxial compressive stress against strain for hcp are non-linear and vary somewhat with the strain rate. Caution is therefore needed in comparing values of Young's modulus obtained in different investigations, results obtained a t very low stresses by the dynamic method in which a resonance frequency of vibration is determined being higher than ones given
270
Cement Chemistry
by static or slow loading methods and averaged over greater ranges of stress. Using the dynamic nlethod, Helmuth and Turk (H45) obtained values of 20 000-30 000 MPa for saturated, mature pastes. Trends with wjc and degrce of hydration, and relations with porosity, were similar to thosc for compressive strength. For saturated pastes, Helnluth and Turk found
where E is Young's modulus, E<,is its value extrapolated to ~ e r oporosit!,. and p, is the capillary porosity. For two cements, Hclmutli and Turk f o ~ ~ n d E, to be approximately 30 000 MPa. The same equation held eqilally well for total water porosity if values around 75 000 MPa were used for E,,. They also found (H46) that, for a series of pastes 6 7 months old. the irrc\.crsible shrinkage at 47% RH was linearly related to the total water porosit!.. whereas the reversible shrinkage was practically independent of the porosity. Sereda P I 01. (S79) found that, as with conlpressive strength. thc values obtained depend on the RH a t which the sample is conditioned. but that the effect is in the opposite direction, saturated pastes having higher moduli than dried pastes. This was attributed to stiffening of the structure through entry of H,O molecules into interlayer sites, possibly augmented by stronger bonding between adjacent layers (S79,F32,S85). Hysteresis occurred. and was explained by assuming that entry and exit of water begins at the edges of the layers, and that the modulus depends primarily on whether thc central parts of the layers are filled. Compacts made by compressing bottle hydrated cement at 100-750 MPa gave results similar to those for pastes.
8.5.2
Drying shrinkage
Cement pastes expand slightly during wet curing; a saturated paste shrinks on being dried. For a mature paste, the linear shrinkage on strong drying is typically somewhat under 1%. The shrinkage on first drying is partly irreversible. Further wetting and drying cycles lead to some increase in the irreversible shrinkage, but after a few such cycles the variation with changes in R H is essentially reversible. A small shrinkage, called autogenous shrinkage, occurs on sealed curing. The changes in length, or more properly in volume, broadly parallel ones in water content, but the parallelism is not exact (F13). Shrinkage is complicated by the effects of moisture gradients in the specimen, and to minimize these and the resulting mechanical constraints, it is necessary to study thin slices of material. Wittmann (W24) obtained data for specimens of varying thickness and extrapolated to zero thickness.
Structure and Properties of Portland Cement Pastes
271
At least four effects might contribute to reversible shrinkage: Capillary stress
Attractive forces exist between the water in a pore and the solid surfaces; when the liquid evaporates, the tension in the meniscus is transferred to the walls, and the pore tends to shrink. The shrinkage is not necessarily reversed when all the liquid has gone, because the pore may collapse. This erect could be important between about 90% and 45% RH. Above 90% i t is unlikely to be important because the pores that are being emptied are widc. and the resulting stresses are small, and below about 45%, a stable meniscus cannot form. Surface free energy
Due to unsatisfied bonding forces, the surface of a solid particle is under tension, as in a liquid. Adsorbed molecules decrease thts tension. and if they are removed, the particle tends to contract. The greatest difference occurs when the last adsorbed layer is removed, and the efl'ect should therefore be greatest below about 20% RH. It can only occur to the extent that closer packing is possible within the solid particle, e.g. by rearrangenlent of smallcr units. Although discussed here in relation to a particle, it could prcsumably also occur in a convex portion of an irregularly shaped. continuous structure. Disjoining pressure
If two solid surfaces are in contact and the attractive forces between them are outweighed by those existing with molecules of the liquid, the latter may be drawn in between them, so forcing them apart; a disjoining pressurc is s a d to exist. The effect occurs with many clay minerals. The main attractlvc forces in the latter case are probably ion-dipole forces betwcen ions in the solld surfaces and water molecules, and the same could apply to C S H . Movement of interlayer water
This is essentially the same effect as the last, but occurs between layers within a single particle. There could well be a continuum of effects, ranging from movement of water molecules between o r away from the surfaces of adjacent separate particles to similar movement involving layers within a particle. An intermediate situation in such a continuum might be movement of water into or out of interlayer spaces a t re-entrants at which layers of a continuous structure are splayed apart, such as those shown in Fig. 8.4.
272
Cement Chemistry
There has been no general agreement as to the relative importance of these efrects. Irreversible shrinkage probably results from an ageing process. in which particles or individual layers of the gel form permanent contacts that bond them into a more stable structure. Powers (P34) regarded disjoining pressure as the most important effect at all humidities. Feldman and Sereda ( F 3 l ) considered that capillary stress and surface energy were important at the higher relative humidities, and niovelnent of interlayer water below about 30%. Bentur ct (11. (B54) concluded from a study of calcium silicate pastes in which shrinkage and creep were related to N2sorption isotherms and determinations of average silicate anion size that irreversible shrinkage was due in young pastes to a reduction in pores of 4- 10 nm diameter. but in mature pastes to changes in the C-S-H, but Parrott and Young (P38) found no correlation between silicate polymerization and irreversible shrinkage in mature C,S pastes. They attributed irreversible shrinkage to capillary tension forces and reversible shrinkage to capillary tension and surface energy effects, depending on the RH. Ferraris and Wittmann (F37) considered the dominant effects to be disjoining pressure down to 40% and surface energy below 40%. Hansen (H43) concluded from a study of initial drying shrinkage in relation to weight loss and pore structure determined by MIP and N, sorption that the main source of stress over the entire range of RH was surface energy. Capillary tension was a subsidiary source at RH > 25%. especially at higher w/c ratios. The stresses were largely accommodated by an irreversible decrease in interlayer spacing.
8.5.3
Creep
Creep is slow deformation under load which is superimposed on the elastic strain. I t can reach values several times greater than the latter, and unlike it is only partly reversed if the loud is removed. The irreversible component. which is the major one, occurs on a time scale of months or longer. and the rwersible coniponent on one of days or weeks. Creep taking place at a constant R H is called basic creep. In practice, creep is likely to be accompanied by drying shrinkage, and if the loading is compressive. the resulting deformation is greater than the sum of those shown by two specimens in which the eHects operate separately. The additional deformation is called drying creep. Creep occurs in tension and with other types of stress as well as in compression. Creep decreases with age before loading and with decreasing w/c ratio (V5). If the material is heated at 110•‹Cbefore loading and kept dry during test. creep almost disappears. It reappears on re-exposure to water. In contrast, increase in temperature from 10•‹C to 60•‹C during loading in-
Structure and Prooert~esof Portland Cement Pastes
273
creases creep. Heating before loading causes the irrecoverable component of creep to decrease even if the specimen is kept saturated (P41 .P41). Standing at ordinary temperature. drying at 110•‹C and heating under saturated conditions are all forms of ageing which produce a more stable structure. Irreversible creep may also be a form of ageing in which a siniilar etfect is produced by stress (F38) and which occurs less readily i f a more stable condition has already been reached through some other cause. There is no general agreement on the mechanism of either reversible or irreversible creep, but the latter. at least, probably involves relative niovements of particles of C-S-H or shearing movements of individual layers within particles or both (F32,B54). The presence of water appears to be essential. Bentur ct (11. (B54) suggested that interlayer S i - - 0-Si bonding contributes to irreversible creep. but the evidence for this is weak. Such bonding would, moreover, be unlikely to occur at ordinary temperature, and, if i t did, would probably produce QQetrahedra, which are not found.
8.6 Permeability and diffusion
8.6.1 Permeability to water The permeability of concrete and the rates at which ions and gases diffuse in i t are of ni~ijorimportance for durability. We shall consider only the behaviour of cement paste. Under certain experimental conditions permeability to water obeys d'Arcy's law:
where dq dt is the Row in m \ - ' , X I is the pernieability in m s - ' . .4 is the cross-sectional area in ni', A11 is the head of water in metres, and I 1s the thickness of the specimen in metres. K, depends on the viscosity of the liquid, the temperature and the properties of the material. Important conditions are that the sample is completely saturated, steady Bow is maintained, and osmotic effects, such as might be caused by concentration gradients in dissolved ions, are absent. Feldman (F39) gave a review. including references t o experimental techniques. Powers and co-workers (P20,P34,P43) determined values of K, of hcp for various combinations of w/c and age. They assumed that the capillar! pores in the paste were initially continuous, but that at a certain stage in hydration they became segmented into isolated cavities, so that the water could only
274
Cement Chemistry
travel through the gel pores, causing the permeability to decrease to a very low value. They concluded that the age at which capillaries became discontinuous increased with w/c. For a typical cement, i t was about 3 days at w,c = 0.4. 1 year at w/c = 0.7 and unattainable at w/c > 0.7. Reinterpreting some of these data. Verbeck and Helniuth (V5) showed that the permeability was directly related to the capillary porosity. Subsequent studies have shown that K , is not related to the total water porosity but depends on the volume and connectivity of the larger pores. Mehta and Manmohan (M75) and Nyame and lllston (N16) found linear relations between log K, and estimates of the maximum continuous pore radius, obtained from MIP, and other quantities derived from the pore size distribution and degree of reaction. A high proportion of the flow appears to be through pores wider than about 100 nm. Typical values of log K , for mature pastes cured at ordinary temperatures range from around - 13.4 at w c 0.3 to around - 11.8 at w/c 0.7 (M75,N16,G67,M76,H47). K , increases with temperature (G67,M76). Hooton (H48) noted some pitfalls in the experimental determination of pcrtneabilities and in the practical utilization of the results. In an experimental determination, accurate results will not be obtained unless the sample has been vacuum saturated; and, even if it is, equilibrium flow in accordance with d'Arcy's law may not occur because water is being used up to continue hydration. In practice, a concrete sample is probably not often saturated throughout; if it is not, capillary forces, as well as the pressure difrerence. affect the rate of flow. Applications of results obtained with pastes to concrete are further complicated by the presence in the latter of cracks. poorly compacted areas and other inhomogeneities.
8.6.2 Diffusion of ions and gases
Ushiyama and Goto (U 15) studied the diffusion of ions in a Portland cement paste of w/c ratio 0.4 cured at 20•‹C for 28 days. Fick's second law was shown to be followed when a steady state had been reached. For Li'. Na' and K'. diffusion coefficients of 1.4-3.3 x lo-'' m2 s - ' were obtained. That of C1- was somewhat greater, especially if the balancing cation was Ca2' or Mg2 + . It was shown that CH and MH crystallized at the surface of the specimen, suggesting that CH within the specimen had dissolved. Other studies on Na' and CI- (C40,P44,G68) have given results i n substantial agreement with those above. At ordinary temperatures. the diffusion coefficients obtained have been lo-" to 10-13 m2 s - ' for Na' and 10- " to 10-l2 m2 s- ' for CI-. The activation energies are reported to be 84 kJ mol-' for Naf and 50 kJ mol-' for CI- for wlc ratios of 0.35-0.45
Structure and Properties of Portland Cement Pastes
275
(G68), or 4 2 4 5 kJ mol- ' for CI- at w/c ratios of 0.4-0.5 and 32 kJ mol- ' for w/c = 0.6 (P44). These activation energies are much higher than those found for diffusion of the same ions in a dilute solution, which are typically below 20 kJ mol-I. For w/c ratios of 0.35-0.45 neither the diffusion coefficient nor the activation energy varies significantly with either age or w/c ratio (G68). These observations, which contrast markedly with those on permeability, suggest that diffusion occurs by movement of the ions on the surfaces of the hydration products, or, at least, that the mechanism involves strong interactions with them (P44,G68). At w/c ratio 0.6, the lower activation energy could arise from the presence of relatively large and unconstricted capillaries not present at lower w/c ratios, which could A, I I ow the rate-limiting step to be more easily overcome (P44). Studies on the permeability of concrete to gases, reviewed by Feldman (F39), have provided only slight information regarding the mechanism of transport through the cement paste or the relations to its pore structure. Oxgyen permeabilities of Portland cement concrete equilibrated at 55% R H are typically 10-l4 to lo-'' m2 s-I, and are strongly correlated inversely with compressive strength and positively with oxygen diffusivity (L45). They increase very markedly on drying, and it cannot be assumed that their relation to pore structure is the same as that for water permeability.
-
Composite cements 9.1
Introduction
A composite cement is a hydraulic cement composed of Portland cement and one or more inorganic materials that take part in the hydration reactions and thereby make a substantial contribution to the hydration product. This definition excludes admixtures, such as CaCI2. that influence the hydration process but d o not themselves contribute substantially to the product. The inorganic materials will be called mineral additions. The most important are pulverized-fuel ash (fly ash; pfa), ground granulated blast furnace slag (ggbfs), natural pozzolanas and microsilica (condensed silica fume). The mineral addition may be ground together with the cement clinker and gypsum, or mixed with Portland cement when the latter is used. In British usage, these procedures are called intergrinding and blending, respectively. Complete cements are used for various reasons. The necessity for utilizing waste materials and decreasing overall energy consumption is becoming increasingly obvious. Pfa and slag are waste materials produced in large quantities, and concretes made with them, or with natural pozzolanas, can have similar properties to ones made with pure Portland cements at lower cost per unit volume. For the cement manufacturer, intergrinding provides a degree of flexibility in adjusting the volunle of cement production to changing demand. Concretes made using composite cements can have properties that are desirable for particular purposes. such as slower and decreased total heat evolution in massive structures, improved durability or, with microsilica, strengths above the normal range. Mineral additions may be broadly categorized as pozzolanic materials or latent hydraulic cements. Neither type reacts significantly with water at ordinary temperatures in the absence of other substances. Pozzolanic materials are high in SiO, and often also in A1,0,, and low in CaO; they are sufficiently reactive that mixtures of them with water and CaO produce CS-H at ordinary temperatures and thereby act as hydraulic cements. If they contain AI,O,, calcium aluminate or aluminate silicate hydrates are also formed. Because they are low in CaO, this component must be supplied in stoichiometric quantity. In a composite cement, it is provided by the Portland cement through decreased formation of C H and decreased Ca/Si
Composite Cements
277
ratio of the C-S-H. Pfa low in CaO, natural pozzolanas and microsilica art: examples of pozzolanic materials. Latent hydraulic cements have compositions broadly intermediate between those of pozzolanic materials and Portland cement. They act as hydraulic cements if mixed with water and a minimal amount of some other substances that serves as a catalyst or activator. Ggbfs is a latent hydraulic cement. When mixed with Portland cement, it is activated by the CH and alkali which the latter produces. As will be seen, the hydration products of the Portland cement clinker are also modified. The proportion of a latent hydraulic cement in a composite cement can be higher than that of a pozzolanic material. Some materials, such as finely ground CaCO,, though used in varying proportions and having neither pozzolanic nor latent hydraulic properties, are conveniently considered along with mineral additions. In this chapter and elsewhere, w/s denotes the ratio w/(c + p ) and 'percentage replacement' the quantity IOOp/(c p), where H,, c and p are the masses of water, Portland cement and mineral addition, respectively.
+
9.2
9.2.1
Blastfurnace slag Formation, treatment and use in composite cements
Blastfurnace slag is formed as a liquid at 1350-1550•‹C in the manufacture of iron; limestone reacts with materials rich in SiO, and AI,O, associated with the ore or present in ash from the coke. If allowed to cool slowly. it crystallizes to give a material having virtually no cementing properties. I f cooled sufficiently rapidly to below 80O0C, it forms a glass which is a latent hydraulic cement. Cooling is most often effected by spraying droplets of the molten slag with high-pressure jets of water. This gives a wet, sandy material which when dried and ground is called ground granulated blastfurnace slag and often contains over 95% of glass. In an alternative treatment, called pelletization, the molten slag is partially cooled with water and flung into the air by a rotating drum. The resulting pellets vary in size from a few millimetres to around 15 mm. The proportion of glass decreases with increasing pellet size. Pelletization has advantages to the steel manufacturer of lower capital cost, decreased emission of sulphurous gases, and formation of a drier product, which can also be sold as a lightweight aggregate, but the glass content of the size fraction used to make composite cements can be as low as 50%. The composition of the slag must be controlled within relatively narrow
278
Cement Chemistry
limits to ensure satisfactory and economic operation of the blastfurnace, and depends on that of the ore. It therefore varies considerably between plants. but that of a given plant is unlikely to vary much unless the soilrce of the ore is changed. Table 9.1 gives ranges and means for slags typical of those produced in western Europe. For a wider range of steel-producing countries. the contents of major componcnts are: MgO, 0 21'70; A120,, 5-33Y0; S i 0 2 . 37-42O/0; and CaO, 30-50% (S86). Table 9.1
Chemical compositions of some blastfurnace slags" (D35) Mean
Na,O Mg o A1203
SiOIh PZO, SO, K ?O CaO
Mean Minimum Maximum
Minimum Maximum TiO, MnO FeO S2-
0.55 0.64 1.24 0.94 F0.16 CI 0.019 Ign. loss 0.42 Total' 99.68
0.49 0.34 0.29 0.68 0.06 0.003 0.00
0.65 1.31 9.32 1.25 0.3 1 0.050 1.04
"Means. minima and maxima for 27 slags reprcscntative of French and Luxembourg production in 1980. 'Total SO,: insoluble residue, mean. 0.41'%,.tnininiuni. 0.00%. maximum. 1.32'X. 'Corrected for 0 2 -equivalent to S 2 . F - and CI-.
In composite cements, the granulated o r pelletized slag is blended or interground with Portland cement or clinker. Ternary mixes containing also. for example, natural pozzolanas are used in some countries. Slags vary considerably in grindability, but are usually harder to grind than clinker. Intergrinding may therefore produce a cement in which the slag is too coarsely, and the clinker too finely, ground. For this and other reasons, there can be a case for separate grinding, at least with some combinations of clinker and slag (S87). The relative proportions of clinker and slag vary widely; the content of slag can be over 80%, but up to 45% is more usual. Specifications and terminology based on them vary in different countries; in the U K , Portland blastfurnace cements may contain up to 65% of slag. In this chapter, unless otherwise stated, the term 'slag' is used to mean granulated or pelletized blastfurnace slag and 'slag cement' to mean composite cements consisting essentially of these materials with Portland cement or clinker. In these cements, the slag reacts considerably more slowly than the alite, and strength development is therefore slower to an extent that increases with the proportion of slag. For equal 28-day strengths, replacement of a Portland cement by one containing 65% of slag can lower the
Composite Cements
279
compressive strength by almost half at 2 days, but increase i t by about 12% at 91 days (S86). Workability is similar to that obtained with Portland cements. The rate of heat evolution at early ages and the total heat evolution are both reduced. Fuller comparisons would have to take account of the proportion of slag, the particle size distributions of clinker and slag. and the fact that slags vary in reactivity.
9.2.2
Factors affecting suitability for use in a composite cement
The suitability of a slag for use in a composite cement depends primarily on its reactivity, though grindability and contents of water and of undesirable components, especially chloride, must also be considered. Reactivity most obviously depends on bulk composition, glass content and fineness of grinding. though these are probably not the only factors and the relations with composition and glass content are complex. Several investigators have found that the presence of a small proportion of crystalline material finely dispersed in the glass improves grindability or reactivity or both (S86). Demoulian cr 01. (D35) found that the highest strengths at 2-28 days were given by slags containing about 5% of crystalline phases and that increase in the content of the latter up to 35% produced only small decreases in strength. They attributed the beneficial effect of small contents of crystalline phases to mechanical stress in the glass and provision of nucleation sites for hydration products, and the smallness of the decrease at high contents to a consequent change in the glass composition. which through the separation of merwinite was enriched in aluminium. Smolczyk (S86) reviewed relations between composition and hydraulic properties. Many attempts have been made to assess slags on the basis of moduli based on bulk composition, of which (CaO + MgO + AI,0,)/Si02 is one of the simplest and most widely used. Minimum values for this ratio. such as 1.0 in West Germany, have been incorporated into national specifications. This and similar moduli express the fact that hydraulic activity is broadly favoured by more basic composition, but the effect of AI,O, content is complex and none of the proposed moduli has proved valid for the detailed comparison of slags other than ones of relatively similar compositions produced within a given plant. A regression analysis of compressive strengths on compositions for composite cements made using a wide range of west European slags (S88) showed that increase in AI20, content above 13% tended to increase early strengths but to decrease late strengths. MgO in amounts up to I I% was quantitatively equivalent to CaO. Minor components were found to have important effects: that of MnO was always negative, but those of P 2 0 , and of alkalis were more complex.
280
Cement Chemistry
9.2.3 X-ray diffraction and microstructure of slags Regourd (R34) reviewed structural and other aspects of slags. X R D patterns show an asymmetric, diffuse band from the glass peaking at about 31 28 (CuK,; r/ = 0.29nm) and extending from about 20" to about 37 and a weaker band at about 48" 20 (0.19 nm). Crystalline phases, if prcsent in sufficient quantity, give superimposed, sharper peaks; melilite and mcnvinite are the most usual. Neither periclase nor lime is found and, since i t is present either in the glass or in these inert, crystalline phases. MgO is not a potential cause of expansion, as it may be in a clinker (S89). Glass contents have been determined by light microscopy and by difference from QXDA determinations of crystalline phases (S9O,R34.R35.U 16). Investigators have differed as to how well the results of the two methods agree. Both will fail if the individual crystalline regions arc below a certain size, which is much smaller for QXDA than for light n~icroscopy.but light microscopy complements QXDA by providing microstructural information and does not depend on a possibly uncertain choice of reference standards. Image analysis with the SEM is a further possible method. Individual grains of ground granulated o r pelletized slag are markedly angular. The melilite and merwinite occur as inclusions in the glass. ranging in size from ones easily detectable by light n~icroscopyto dendritic growths in which the individual dendrites are less than 100 nm wide (S90,D35). The melilite crystals may be zoned (U16). Metallic iron can occur. Crystalline inclusions and gas-filled pores can be very unevenly distributed; even in slags high in crystalline material, grains consisting only of glass may be seen using light microscopy (D35). In slags of high glass content, the composition of the glass is reported to be uniform, but near to melilite crystals it can be lower in aluminium ( U 16). Some workers using small angle X-ray scattering and l ~ g h tand electron microscopy have found evidence of phase separation within the glass (S90,R34) but others have concluded that this does not occur (U16): slag glasses may vary in this respect.
9.2.4 Internal structures of slag glasses The internal structures of slag glasses have been widely discussed in terms of the classical theory of network-forming and network-modifying elements. On this model, the glass structure is based on a continuous, though incompletely connected, anionic network composed of oxygen, silicon and the other relatively electronegative elements, and its charge is balanced by the calcium and other electropositive elements. Consideration of the compo-
Composite Cements
281
sitions of typical slag glasses (Table 9.2) suggests that this model, which has proved valuable for more acidic glasses, may be unrealistic for slags. Some important atomic ratios, derived from the data in Table 9.2, are: O/Si = 4.4; O/(Si + Al) = 3.0; O/(Si + Al + P + Mg Fe Mn) = 2.5. These values indicate that nothing resembling a continuous network can be formed by the silicon, aluminium and oxygen atoms alone. Dron and Brivot's (D36) model, in which the network-forming atoms are present in straight or branched chains of various lengths, is more realistic. Studies of six slags using "Si NMR and IR spectroscopy, TMS and an alkali extraction method showed the predominant silicate species to be in all cases monomer and dimer ( U 16). The bulk con~positionswere all near that in Table 9.1.
+
+
Table 9.2 Atoni ratios in a typical ground granulated blastfurnncu slag
K+
N ~ +ca'+M ~ ' +F ~ ' + ~ ~ ' t ~ iA IaM+ ti&+P S +
0.6
0.5
30.7 6.1
0.7
0.3
0.3
10.6 22.4 0.1
t70r the mean of the analyses in Table 9.1; referred to 100 ( 0 ' + S'
02
-
98.5
+F
SZ-
F-
1.2
0.3
)
Evidence on the coordination of aluminium and magnesium in slag glasses is conflicting. From molar refractivities and other evidence. Chopra and Taneja (C41) concluded that both the aluminium and the magnesium are tetrahedrally coordinated; this is the situation in melilites. X-ray spectroscopic results have been held to favour tetrahedral coordination of the aluminium ((369) or a mixture of tetrahedral and octahedral coordination (Y7). From electron paramagnetic resonance, Royak and Chkolnik (R36) concluded that magnesium, aluminium and titanium could each be in either tetrahedral or octahedral coordination and that increased proportions in octahedral coordination enhanced hydraulic activity. Structural defects affect reactivity; four samples of the same slag liquid. cooled at different rates, differed in reactivity, though all were shown by XRD to be completely glassy (F40). Photoelectric emission data showed that they contained different numbers of defects. Granulated or pelletized slags are materials formed under markedly nonequilibrium conditions, and it is not surprising that their hydraulic activities are not determined solely by bulk composition and glass content. Reactivity may be presumed to depend on glass composition rather than on bulk composition. For a given combination of glass content and bulk composition, further effects varying reactivity could thus include the following: (1) Crystalline size and morphology, through their effects on stress in the glass or nucleation sites for hydrated phases. (2) The bulk composition of the crystalline material, through its effect on that of the glass, and the profile of compositional variation in the glass
282
Cement Chemistry
near to the crystalline surfaces, which could produce regions of either enhanced o r decreased reactivity. (3) The degree of phase separation within the glass. (4) The coordination numbers of the aluminium and magnesium. and the distributions of atoms and bonding patterns within the glass, e.g. variation in the number of Si-0-AI linkages compared with those of Si-0-Si and AI-0-AI. (5) The numbers and types of electronic or structural defects.
9.2.5
Hydration chemistry of slag cements
Daimon (D37), Regourd (R37) and Uchikawa (U17) have reviewed slag cement hydration. Many X R D studies have shown that the principal hydration products are essentially similar to those given by pure Portland cements, but the quantities of C H found by this or other methods are in varying degrees lower than those which would be given by the Portland cement constituent if the slag took no part in the reaction (S91,C42,K42,R37). With percentage replacements above about 6OnA, the C H content may pass through a maximum and then decrease. With slags high in MgO, a hydrotalcite-type phase has been detected (K43). The microstructures of slag cement pastes are also essentially similar to those of pure Portland cement pastes, apart from the lower CH contents (R34,U 12,U 17,U l8,H49). As with the clinker phases, layers of in situ reaction product form at the boundaries of the slag grains and gradually extend inwards. A light microscopic study indicated that these were only 0.5 pm wide after 90 days (K42). Backscattered electron images with another slag (H49) showed them to be barely detectable at 28 days, but up to 10 pm wide at 6 months and up to 15 pm wide at 14 months. In the older pastes, fully reacted relicts of slag particles were abundant. The quantitative determination of unreacted slag has proved difficult. Luke and Glasser (L27,L46) made a comparative study of methods described in the literature. They found the most satisfactory to be a modification of that of Demoulian er al. (D38), in which constituents other than unreacted slag were dissolved using an alkaline solution of EDTA in aqueous triethanolamine. Other extraction methods gave unacceptably high residues with pastes of Portland cements. The EDTA solution did not dissolve hydrotalcite; this is likely to make the results for unreacted slag high. The scanty results in the literature for percentages of the slag that has reacted (Table 9.3) show wide discrepancies. The extents to which these result from differences between slags, experimental errors, proportions in the
Table 9.3 Percentages of slag reacted in slag cements A. As percentage of initial weight of slag
B. As percentage of total ignited weight
Reference % Slag in
the cement Age (days): 0.25 1 3 7 28 90 180 365 540 Determined by the method of K o n d o atid O h s a w ( K 4 2 ) . w t h correction for other phases undissolved except for Ref. L27, for which a n EDTA ~iicthodwas used.
284
Cement Chemistry
cement or other causes are not clear. The data are somewhat more consistent if expressed as percentages on the total ignited weight than us ones on the initial weight of slag. This suggests that. for a given slag, the rate of reaction may be limited by the rate at which the necessary ions are supplied by the clinker or taken up by its hydration products. Because the sulphur, iron and manganese in a slag are present in lower oxidation states, T G curves or determinations of ignition loss on slag cement pastes must be carried out in oxygen-lice atmospheres. The S 2 - ion is ~xcsentin tlie glass. and its behaviour on hydration is not understood. Slag cements develop a transient green colour on hydration. which suggests that polysi~lpliidesare formed. The possibility of interactions between SO4?and sulphur in lower oxidation states, either during hydration or on lienting in a neutral atmosplierc, cannot be overlooked. If SO2 is formed in the latter case, i t could signilic;~ntly;iITect the interpretation of a T G or ignition loss dcterminution.
9.2.6 X-ray microanalysis
X-ray niicroanalyses (R26.U 18,H4,H49) and analytical electron microscopy (T44) have indicated Ca/Si ratios of 1.55 1.79 for the C - S H surrounding the clinker grains. Some have also indicated higher contents of aluminium. potassium and sodium. These Ca/Si ratios are generally lower than those of 1.7-2.0 that are characteristic of pure Portland cenient pastes. The material Ihrmed in situ from tlie slag is markedly lower in C a O and higher in aluminium and magnesium. Regourd P I trl. (R26) found CIS 0.9- 1.3, C A 4.6-4.8 and C/M 1.4 3.2 for a 28-day-old paste made with n cement containing 70•‹4 of slag and hydrated at 20 C. Harrisson clt (11. (H4,H49) represented the results of X-ray microanalyses of individual spot analyses in all parts of the microstructure other than unreacted clinker grains on plots of AI/Ca ratio against Si/Ca ratio and of Mg/Ca ratio against AI/Ca ratio (Figs 9.1 and 9.2). If the analyses of the material formed in situ from the slag are excluded, the plot of AI/Ca against Si Ca is broadly similar to those obtained for pure Portland cenient pastes. and may be interpreted in the same way (Section 7.2.5). Taken together, tlie results in the two figures show that the material Ihrmed from the slag has an approxitnately constant Si/Ca ratio of 0.62. and ratios of Mg Ca and of AI/Ca that vary from point to point on a micrometre scale but which are related to each other by the equation shown in Fig. 9.2. This was interpreted (H49) as indicating mixtures in varying proportions of
Composite Cements
285
C-S-H having Si/Ca 0.62, Ai/Ca 0.09, with a hydrotalcite-type phase having AI/Mg 0.38. As with Portland cement pastes, the material described in Fig. 9. I as C-S-H may really be an intimate mixt~lreof the latter with AFm and. to a minor extent, hydrotalcite-type structures. Tanaka 6.1 (11. (T45) partially masked the surfaces of disks of slag glass with gold coatings; the disks were then embedded in cement pastes for various periods. after which they were removed and examined by SEM. This showed that two layers of in situ product were formed. The innermost one had Si/Ca 0.64, AI/Ca 0.32 and Mg/Ca 0.34; the outer one had Si/Ca 0.61. AI/Ca 0.24, Mg/Ca 0.20. The product formed imnlediately outside the original boundary of the slag had Si/Ca 0.48. AI/Ca 0.09. Mg/Ca 0.17. The two layers of in situ product could have been mixtures in difyering proportions of C-S-H, a hydrotalcite-type phase and possibly AFm phasc.
and
Relicts
Fig. 9.1 Si/Ca and AI/Ca atom ratios rroni individual X-ray microanalyses of a slag cement paste (40% slag; w/s = 0.5) hydrated for various times at 25 C. Mg Si < 0.1 5: 0, 28 days; 0, 6 months; 0, 14 months. Mg/Si 2 0.15: 28 days; H. 6 months; 0 , 14 months. Harrisson et (11. (H49).
+,
286
Cement Chemistry
.,
Fig. 9.2 AI/Ca and Mg/Ca atom ratios from individual X-ray microanalyses of the same slag cement paste as in Fig. 9.1 Si/Ca < 0.5: 0 , 28 days; El, 6 months; .:. 14 months. Si,'Ca 2 0.5: 28 days; 6 months; 0 , 14 months. Harrisson (,I ctl. (H49).
+,
9.2.7
Stoichiometry of slag cement hydration
Comparison of the average proportions, compositions and densities of the phases present in the material formed in situ from the slag with correspond-
Composite Cements
287
ing data for the unreacted slag suggests that little net movement of either magnesium or oxygen in or out of the latter occurs when i t reacts; ho\vever. substantial fractions of the aluminium, silicon and calcium are expelled and an equivalent amount of hydrogen taken up (H49). The ions thus released contribute to the hydration products formed in the initially water-filled space, but more calcium is needed t o alter the ratios to those in a mixture of C-S--H and AFni phase. This is provided partly at the expense of C H formation, and partly through decrease in the Ca/Si ratio of all the C S H. including that formed in situ from the alite and belite. On this busis. given also the CH content and estimates of the aniounts of clinker phases and slag that have reacted, one may calculate a mass balance table by a simple extension of the procedure described in Section 7.3.3. Water contents, volume percentages of phases and, by diff'erence, porosities. may similarly he calculated. Table 9.4 gives results of these calculations for the cement to which Figs 9.1 and 9.2 relate. For any specified drying condition. the calculated water contents arc lower and the porosities higher than those of pure Portland cement pastes. and this appears to be true in varying degrees of composite cements in general. Experimental observations support this conclusion. N o w e u p o r able water contents of 2-year-old pastes of w/s ratio 0.5 typically cfecrease with slag content from around 23% for pure Portland ccnients to 10 I30L for cements with 90% of slag (C42). For the paste to which Table 9.1 rekrs, the observed non-evaporable water content was 17.7'%1(H49). I'orosities and their relations to physical properties are discussed in Section 9.7.
9.2.8 Activation of slag glasses If ground granulated or pelletized blastfurnace slag is placed in watcr ~tlonc. it dissolves to a small extent, but a protective film deficient in Ca" is quickly formed, and inhibits further reaction (K20.R39). Reaction continue> if the pH is kept suficiently high. The pore solution o f a I'ortlund cement. \\hich is essentially one of alkali hydroxides, is a suitable medium. The suppl! of K ' and Na' ions is limited. but these ions are only partially taken up by the hydration products, and the presence of solid C H ensures that the supply ol' O H is maintained. The slag can be rendered active by OH- ions supplied in other ways, e.g. from calcium or sodium hydroxide, sodium silicate or sodium carbonate (R34). Although several hypotheses have been proposed. the mechanism of the attack on the slag glass is not established. If it is similar to that suggested for the clinker phases (Section 4.5.1) one would expect reactivity to increase with the fraction of the oxygen atoms bonded only to calcium and a single silicon atom or, especially. to calcium and :I single aluminium atom.
Clinker phases
C-S-H
Ca(OH),
AFm
AFt
Slag HP
Fe H P
M g H P CaCO,
Slag glass
Slag res.
Other
Pores
Water conterzts (percer~tageson the ignited weight) B o u n d water: 24.7%
N o n - e v a p o r a b l e water: 17.7%
Porosities Capillary, 17.7%
Free water, 33.3%
T o t a l water, 45.8%
Ratios of water'cement.slag = 0.5:0.6:0.4. Slag glass reacted = 22%. CH content = 9.4%. both referred to the ignited weight of composite cement. Slag HP = in situ product from slag. Fe H P = hydrogarnet-type product from ferrite phase. Mg H P = hydrotalcite-type product, excluding that formed in situ from slag. Slag res. = unreacted crystalline part of slag. 'Other' components mainly P,O,. 'Other' phases mainly insoluble residue and alkalis present in or adsorbed on products or contained in the pore solution. Discrepancies in totals arise from rounding. Results modified from Ref. H39.
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Cement Chemistry
Calcium sulphate accelerates the reaction of slag glasses. probably because precipitation of ettringite provides a sink for the CaZt and AI(OH),- ions released from the slag. I t is often described as an activator, but is not very etrective unless a little alkali is also present (R37) and is perhaps more properly described as a powerful reactant. The calcium sulphate present in a slag cement thus contributes to increasing the activity of the slag. The reaction between slag and CaSO, is utilized to a much greater extent in supersulphated cements.
9.2.9 Supersulphated cements Supersulphated cements consist of 80--850h of slag with 10 15% of anhydrite and about 5% of an activator, which is usually Portland cement clinker, and are somewhat more finely ground than Portland cements. A slag high in AIzO, is preferred. X R D shows that the main hydration products'are C-S H and ettringite. The content of the latter reaches a limit in about 3 days, by which time virtually all the anhydrite has reacted (S90,K42). More water is taken up during curing than with Portland or normal slag cements. and contents of non-evaporable water are higher. TEM examination of replicas (M77) confirmed the formation of ettringite, some crystals of which were over 120 pm long. Kondo and Ohsawa (K42) studied a cement with 80% of slag (18.5% AI,O,), 15% CaSO, and 5% C,S. At 3 days, 91% of the CaSO, and 22% of the slag had reacted, and the content o'f non-evaporable water was 26%. To convert all the SO, supplied by the CaSO, into ettringite, about 19(Y0of the slag would have to react, which agrees well with the observed value. The quantity of ettringite formed would be 43"/0, referred to the ignited weight. The high content of non-evaporable water is difficult to explain. as ettringite normally loses at least a half of its water on being dried under the conditions used; however, XRD indicated that the ettringite formed from the cement was particularly resistant to dehydration, and a little non-evaporable water would also have been contributed by the C-S-H. Daimon (D37) reported broadly similar results.
9.3 Pulverized fuel ash (pfa; fly ash) l o w in CaO 9.3.1 Properties Pfa is ash separated from the flue gas of a power station burning pulverized coal. It must be distinguished from the coarser ash that collects at the bottom of the furnace; the most uniform and highest quality ash is likely to
Composite Cements
291
be that produced by efficient, base-load power stations. Ravina (R40) summarized the technology. The chemical and phase con~positionsdepend on those of the minerals associated with the coal and on the burning conditions. In general, anthracitic or bituminous coals give ashes high in glass, SiO,, A1,0, and Fe,O, and low in CaO, whereas sub-bituminous coals or lignites give ashes higher in CaO and often also in crystalline phases. The pfa used in Europe and Japan to make composite cements typically contains under 10% of CaO. The American designations of Class F and Fe,O, above Class C fly ash, though based on contents of SiO, + A1,0, and below 70%, respectively, correspond approximately to low- and highCaO ashes in the sense used here. Section 9.3 deals exclusively with pfa containing less than about 10% of CaO. Kokubu (K44) and Regourd (R34) reviewed the major characteristics of pfa. Others (W27,T46,W28,H50) have described the pfa produced in the UK more fully. Table 9.5 gives data for a good-quality ash. SEM (e.g. Ref. R41) shows such pfa to contain a high proportion of spherical and largely glassy particles, formed by the rapid cooling of droplets of liquid. Etching with HF (R41) or partial hydration (HSI) reveals inclusions of prisms of mullite and smaller crystals of quartz, which are less reactive. Hollow spheres. called cenospheres, occur; ones that contain smaller spheres are called plerospheres. K,SO,, CaSO,, hematite (Fe,O,) and magnetite (Fe,O,) may be present as smaller particles adhering to the surfaces of the spheres. The iron oxides and quartz also occur as separate, angular particles. Carbon. if present in sufficient amounts, forms porous particles that can be ei~her spherical or irregular in shape (R41). For the more finely divided ashes, 50% by weight is in particles smaller than about 10 pm; for the coarser ones. the corresponding size is around 40 pm.
+
Table 9.5 Chemical and phase compositions and density of a typical pfa low in CaO and high in glass (W27)
Na,O
MgO A120, SiO, P,O, SO, K,O CaO TiO, Mn20, Fe,O, C
1.5
1.6
27.9
48.7 0.2
1.2
4.2
2.4
0.9
tr.
9.5
H,O Total
1.5 0.3
Quartz
Mullite
Hematite
Magnetite
Carbon
Glass"
Density (kgm-,)
2.8
6.5
1.6
1.9
1.5
86
2220
" By dill'erence.
99.9
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Cement Chemistry
XRD patterns of pfa include a diffuse band from the glass, which for ashes low in CaO peaks at 22-23" 20 (CuK,). This is slightly above the value for vitreous silica. Possibly in all such ashes, the glass shows phase separation on a 5-15-nm scale (QI). The proportion of glass and other amorphous aluminosilicates, if any, may be determined by difference from QXDA on the superimposed peaks of the crystalline phases (D 12); allowance should be made for the content of amorphous carbon. The same method shows that. in many ashes, the FeO and Fe203 are mainly present as magnetite and hematite. Hubbard ct al. (H50) found a correlation between the KzO/ AI,O, ratio and the percentage of amorphous aluminosilicates. Since it is overwhelmingly the latter that take part in the pozzolanic reaction. they suggested that this ratio, multiplied by 10, might be used as a pozzolanic potential index (PPI). Due to the presence of hollow spheres and spongy material, the density of pfa is difficult to define. The smaller spheres are not readily broken by grinding, and values of 2100-2400kgm-3, increasing with iron oxide content, are typical (K44), but these must be influenced by the presence of carbon and voids, as AI,03-SiO, glasses have densities of 25002700 kgrn-, and the densities of the crystalline constituents range from 2650 kg m - 3 (quartz) to 5240 kg m - 3 (hematite). Mather (M78) found that fine grinding of a particular pfa caused the density to increase from 2440 to 2780 kg m-3. Watt and Thorne (W27) found that the weight distributions of density for five pfas all peaked at 2500-2600 kg m-,, and this is probably a reasonable estimate of the true density of the reactive part of the material.
9.3.2
Factors governing suitability for use in composite cements
Important properties governing the suitability of a pfa for use in a composite cement are the content of unburned carbon, the ability to decrease the water demand of the concrete and the pozzolanic activity. The second property partly depends on the first. An excessive content of carbon also produces a discoloured concrete and interferes with the action of some concrete admixtures, especially air-entraining agents. The effect on water demand is important because the pozzolanic reaction of the pfa is slow, and to obtain a specified 28-day strength, the w/s ratio must be lower than for an otherwise similar mix containing a pure Portland cement. Early work, reviewed by Kokubu (K44), established that partial replacement of cement by a goodquality pfa increases the workability. Typically, to produce a concrete with the same 28-day strength and the same slump as one made using a pure Portland cement and the same coarse and fine aggregate, some of the cement and some of the fine aggregate are replaced by pfa and the content of water per unit volume of concrete is decreased.
Composite Cements
293
The enhanced workability of concrete containing good-quality pfa has generally been attributed to the fact that the latter is largely composed of smooth, spherical particles (K44), together with the increased proportion of paste and the decreased formation at this stage of hydration products. Uchikawa (U17) reviewed rheological studies. Pfa containing a high proportion of coarse ( > 45 p)material is unsuitable for blending but may be suitable for intergrinding, because the effect of the latter is largely to scpnrate particles present in agglomerates and to break up some of the larger cenospheres; the spherical particles are not otherwise damaged (M79). Chemical tests of pozzolanicity have proved of limited use for evaluation, because the 28-day strength depends much more on the w/s ratio than on this property, and there appears to be no effective substitute for direct testing of the relevant properties on mortars or, preferably, on concretes. The degree of pozzolanic activity is, however, important for the development of strength and decrease in permeability at later ages. For equal 28-day strengths, the strengths at 91 days or more normally exceed those of otherwise similar concretes made with pure Portland cements.
9.3.3
Rates of consumption of clinker phases and pfa, and contents of calcium hydroxide
Uchikawa (U 17) reviewed the hydration chemistry of pfa and other composite cements. Pfa cements differ from pure Portland cements notably in (i) the hydration rates of the clinker phases, (ii) CH contents, which are lowered both by the dilution of the clinker by pfa and by the pozzolanic reaction. (iii) the compositions of the clinker hydration products and (iv) formation of hydration products from the pfa. The two last aspects cannot be wholly separated. Calorimetric studies show that pfa retards the reaction of alite in the early stage of reaction (G70,J30,HSl,W29), but with one exception (W29) studies on the middle stage show the alite reaction to be accelerated (K45,L47.H51,T44,DI2). SEM shows that C-S-H and CH are deposited on the surfaces of pfa grains before these have started to react significantly (D28). The accelerating effect is probably due mainly or entirely to the provision of additional nucleation sites on the pfa, and occurs with other fine powders (K46). A QXDA study showed that the aluminate and ferrite react more rapidly in the presence of pfa, but with belite there was no detectable effect up to 28 days and a marked reduction in rate of consumption thereafter (D12). Fig. 9.3 shows typical results for the contents of CH in interground cements containing various proportions of pfa. At early ages, the CH content, referred to the mass of clinker, increases with increasing proportion
294
Cement Chemistry
of pfa due to the acceleration of the alite reaction, but at later ages it decreases due to the pozzolanic reaction. The decreases would. of course. be greater if the CH contents were referred to the total ignited weight. including the pfa.
AGE I N DAYS
( S Q U A R E ROOT S C A L E )
Fig. 9.3 Calcium hydroxide contents of pastes of Portland pf21 ccnlcnts conlalnlng 0. 20. 40 and 60% of low-CaO pfa. Taylor ct crl. (T44).
SEM (H51) showed that the glass in one pfa cement was heavily etched after 7 days and that many of the particles up to 1-2 pm in size were consumed within 28 days. The crystalline phases appeared to be inert. A QXDA study of another pfa cement (Dl 2) confirmed an earlier report (K44) that the ratio of mullite to quartz does not change on hydration. and supported the view that the crystalline phases in pfa do not react significantly within I year. In contrast, a TEM study (R42) of a I-year-old specimen indicated that some, at least, of the quartz reacts at a rate comparable to that of the glass. Quantitative data on the rate of consumption of pfa are few and somewhat variable. Those based on differences between the CH contents of pure Portland and pfa cements are suspect, because the calculation involves the effects of pfa substitution both on the rate of consumption of the clinker phases and on the compositions of the products, which are not fully understood. Unreacted pfa has been directly determined by dissolution of the other phases with HCI (C43) or with salicylic acid in methanol followed by HCI (T44), chemical separation of the residual pfa followed by QXDA determination of its content of crystalline phases (D12) and a trirnethylsilylation method (U19). A method based on EDTA extraction was found unsatisfactory (L46).
Composite Cements
295
Fig. 9.4 shows results from several investigations on pfa cements at or near 20•‹C.The tentative curves of equal mass of pfa reacted are based on thc more typical of the data. Such curves are necessarily U-shaped if extended over the entire range of pfa content from 0 to 100%. The d~screpancies between the results of different investigations, which are especially marked at the later ages, probably arise partly from experimental errors and partly from differences in the reactivity of the pfa or the conditions under wh~chit was used. It is difficult, however, to reconcile the higher values shown with the amounts of CH present on any reasonable assumptions concerning the stoichiometry (Section 9.3.6).
PERCENTAGE REPLACEMENT O F CEMENT
BY
PFA
Fig. 9.4 Degrees of reaction of low-CaO pfa in pastes of Portland-pl: ccmcnts hydrated at 15-25•‹C. Numbers against data points denote kg of pfa rcacwd per 100 kg of composite cement at the age and percentage of pCa in the composite ccrncnt indicated. The curves of equal quantity of pfa reacting arc based on thc more (!pica1 of the results. Sources of data: O (K45,K47); 0 (T44); (UIV,U17): 0 (CJi): 0 (D 1 2).
Of the pfa characteristics that influence reactivity, the glass content appears to be much the most important, but specific surface area. glass composition and the effect of stress in the glass caused by the crystalline inclusions may also be relevant (U 17). Of external factors, the RH. temperature (C43) and alkali content of the cement are probably the most important. Sulphate ion may also enhance reactivity by promoting the removal of A13+ from the glass (U17). The rates of the pozzolanic reaction and of strength development are more sensitive to temperature than are those of hydration and strength development for pure Portland cements (e.g. Ref. H52).
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Cement Chemistry
9.3.4 Microstructure and compositions of the hydration products
Backscattered electron and X-ray images obtained in the SEM (021,U 17,S68) show that the microstructures of hardened pastes of cement or C,S with pfa broadly resemble those of pure Portland cements, though, as would be expected, there is considerably less CH. Reaction rims may be seen around the pfa particles in sufficiently old pastes. The hydrated material has been analysed by X-ray microanalysis and analytical electron microscopy. In a 3-day old paste, that formed in situ from alite or belite did not differ significantly in composition from the corresponding product in pure Portland cement pastes (H4). but at later ages Ca: Si is lower and AI/Ca higher (R25,R26,T44,U 17,U 18,R42). Ca/Si is typically about 1.55, but the value decreases with age and ratio of pfa to clinker. Uchikawa (U20,U17) reported a value of 1.01 for a 4-year-old paste with 40% replacement of cement by pfa. Several of the studies (R25,T44,U20,U 17) showed that the C-S-H was higher in alkalis if pFii was present, but one cannot tell to what extent potassium or sodium apparently present in the C-S-H has been deposited from the pore solution on drying. For material close to the pfa particles in a 10-year-old mortar. Sato and Furuhashi (S92) found a Ca/Si ratio of 1.1-1.2. For the microstructure as a whole, a general impression of compositions can be obtained by plotting the AI/Ca ratios of individual spot analyses against the Si/Ca ratios, as for Portland or slag cement pastes (Fig. 9.5: compare Figs 7.5 and 9.1). For the paste to which Fig. 9.5 relates. the inner product of the alite or belite had mean Si/Ca = 0.61 (Ca/Si 1.63) and mean AI/Ca = 0.10, with small contents of sodium, magnesium, sulphur, potassium and iron (H4). In reflection, the space resolution does not allow reliable analyses of the material formed in situ from the pfa to be obtained. Material near to the pfa particles was often found to include CH; where this was not present, the analyses suggested mean Si/Ca 2 0.62 and mean AI,'Ca 1 0.18. The analyses in this group were notably high in potassium and sulphur ( K ; Ca 0.04-0.06, S/Ca 0.04-0.07). Some relatively large crystals of AFm phase found in a 6-month-old paste had compositions broadly intermediate between monosulphate, monosilicate and C,AH,. Rodger and Groves (R42) studied ion-thinned sections of a C,S-pfa paste by TEM, including microanalysis and diffraction. Most of the unreacted pfa particles consisted of impure alumina-silica glass, sometimes with inclusions of mullite. Some were of nearly pure silica glass, with or without quartz. or of iron oxides. In a I-year-old paste, a typical alumina-silica particle about 12 pm in diameter had developed a reacted zone some 800 nm thick and consisting largely of radially fibrillar material described as C-S-H. with denser regions of hydrogarnet and a denser, outer rim of C-S-H. The
Composite Cements
297
hydrogarnet had a cell parameter of 1.24nm and conlposition C,2A,FS,H,,. The C-S-H in the reacted zone had Si/Ca 0.70. Al/Ca 0.20 and Fe/Ca 0.14; that in the rim had Si/Ca 0.68, AI/Ca 0.09 and Fe/Ca 0.05. Both contained small amounts of potassium, sodium, magnesium, titanium, sulphur and other elements. Surprisingly, hydrogarnet was also observed in the reacted zones of pfa particles consisting of almost pure silica. It was suggested that these provided favourable nucleation sites, as the aluminium and iron must have come from outside. Concentric bands of differing apparent density within the reacted zones were sometimes observed. and had possibly formed by a Liesegang ring mechanism.
"0
0.2
0.4 SilCa
0.6
0.8
Fig. 9.5 SilCa and Al/Ca atom ratios from individual X-ray microanalyses ofa 91day-old paste of a Portland-pfa cement (28% of a pfa low in CaO; wjs = 0.5). Open circles, in situ product from alite or belite; open squares, regions near to pfa particles. and in some cases also to Ca(OH),; full circles, undesignated product. P: suggested composition of in situ product from pfa, possibly composed largely of C-S-H and hydrogarnet (HGT). Modified from Harrisson et 01. (H4).
TMS evidence shows that silicate anion condensation occurs more rapidly in C,S-pfa pastes than in pure C3S pastes. Mohan and Taylor (M80) found that at 1 year, about 60% of the silicon (excluding that in unreacted pfa) was present as polymer in the former case and 40% in the latter. Uchikawa and Furuta (U9) reported data showing ratios of silicon present as polymer to that present as dimer of 0.56 for a pure C3S paste 180 days old and of 0.80
298
Cement Chemistry
for one containing pfa. These results agree with the evidence on C-S-H composition.
9.3.5
The nature of the pozzolanic reaction
The primary reaction of any pozzolanic material is an attack on the SiO, or A120,-SiO, framework by O H - ions. I t may be supposed that the O H ions attach themselves to silicon and other network-forming atoms, with consequent breaking of bonds between the latter and oxygen atoms. After this has occurred several times, the silicate or other oxy anion is detached fronl the framework. I t may either remain in situ or pass into the solution. The charges of those that remain are balanced, partly by H', and partly by metal cations. Since a cement pore solution is essentially one of potassium and sodium hydroxides, the immediate product is likely to be an anlorphous material with K and Na' as the dominant cations, but the more abundant supply of Ca2 and the lower solubility of C-S-H and hydrated calcium aluminate or silicoaluminate phases will ensure that this is only an intermediate product. Its presence is indicated by the relatively high potassium contents observed in or near to the reacting pfa particles. The products formed in situ from the cement and in the initially waterfilled space are modified by the removal of CaO, and the provision of additional SiO, and A120,. The CH content is,lowered, initially because less is formed and later also by dissolution of that already present (Fig. 9.3). There is a decrease in the Ca/Si ratio of the C-S-H, even including that formed in situ from the alite, and additional C-S-H and AFm phase are formed. The products formed in situ from the pozzolanic material are likely to reflect its composition and the situation in which SiO, and AI20, are readily available compared with CaO. One therefore expects to find C--S-H of low Ca/Si ratio. together with stritlingite or hydrogarnet or both. The conipositions represented in Fig. 9.5 could arise from mixtures consisting largely or wholly of C-S-H and a hydrogarnet similar in composition to that found by Rodger and Groves (R42). The constituent of the in situ product described by the latter as C-S-H is unlikely from its high Ca/Si, Al/Ca and Fe/Ca ratios to have been pure C-S-H, but might have been a very close mixture of a C-S-H of lower Ca/Si ratio with other phases high in aluminium or iron. +
+
9.3.6
Stoichiometry of pfa cement hydration
The relevant data for pfa cements are too scanty and too uncertain to allow
Comaos~teCements
299
more than a tentative approach to the hydration stoichiometry. Table 9.6 gives results calc~~lated for the paste to which the data in Fig. 9.5 apply. The CH content and quantity of pfa glass reacted were estimates based on Figs 9.3 and 9.4, respectively, and calculation was by a modification of the method described in Section 7.3.3. It was assumed that the product formed in situ from the pfa had the composition given by the X-ray microanalyses and that its amount was such that the number of oxygen atoms per mi nit volunie did not change when i t was formed. As with pastes of pure Portland or slag cements, it is uncertain how much, if any, of the aluminium and other elenlents are true substituents in the C-S-H and how much are present in phases closely admixed with it. An AI/Si ratio of 0.05 for the C-S-H was arbitrarily assumed. As with the slag cement discussed in Section 9.2.7, the calculated water contents for different humidity states are lower, and the porosities higher than for conlparable Portland cement pastes. The water contents are lower because replacement of C H by C-S-H or hydrated aluminate phases causes relatively little change in the H,O/Ca ratio, so that the water content tends to fall towards the value which would be given by the Portland cement constituent alone. Diamond and Lopez-Flores (D39) found that, for two 90day-old pastes similar to that under discussion (30% pfa, w/s = 0.5). the non-evaporable water contents were 12.5% and 13.0%. while that of a Portland cement paste was 15.4%. The porosities are discussed in Section 9.7.
9.4 Natural pozzolanas 9.4.1
Properties
Most natural pozzolanas are of volcanic origin, though some arc sedimentary. Some clays and other materials that are unsuitable for use in concrcte in their natural state become usable as pozzolanas if heat treated. Both natural pozzolanas and heat-treated materials have been used with lime since ancient times, but today they are mainly used as constituents of pozzolanic cements. Several reviews are available (M8 1 ,M82,M83). The volcanic deposits range from unconsolidated materials to ones showing varying degrees of compaction and chemical alteration. The consolidated ones are called tuffs. Not all such materials are pozzolanic; the usual active constituents are glass high in silica and often highly porous, or zeolites, or both. Italian pozzolanas, Santorin Earth (from Greece) and some rhyolites (e.g. from the USA) are examples of poorly consolidated deposits. and trass (from Germany) is one of a tuff. Some of these materials include
Table 9.6 Calculated mass balance and compositions by weight and by volume for a 91-day-old paste of a pfa cement Percentugc.~otz tlze ignited ~ t ~ i g l/or. z r n~uter.iulequilibrated at 11% RH
Na,O Alite Belite Aluminate Ferrite C-S-H Ca(OH), AFm AFt Pfa HP Fe HP Mg HP CaCO, Pfa glass Pfa res. Other Total
MgO
AI,O,
Si0,
SO,
K,O
CaO
TiO,
Mn,O,
Fe,O, Other CO,
H,O
Total
Clinker phases
C-S-H
Ca(OH)2
AFm
AFt
Pfa HP
Fe HP
Mg HP
CaCO,
Pfa glass
Pfa res.
Other
Pores
3.8
21.1
6.8
9.4
5.5
2.3
1.2
1.4
0.4
7.7
2.3
1 .O
37.0
Porosities Capillary, 29.6%
Free water, 37.0%
T o t a l water, 48.0%
Ratios of watericementipfa = 0.5:0.72:0.28.pfa glass reacted = 6.0%. CH content = 13%. both referred to the ignited weight of composite cement. pfa H P = in situ product from Pfa. Fe H P = hydrogarnet-type product from ferrite phase. Mg H P = hydrotalcite-type phase. excluding any present in pfa HP. Pfa res. = unreacted non-vitreous material from Pfa. 'Other' components mainly C and P,O,. 'Other' phases mainly insoluble residue. and alkalis present in or adsorbed on products or contained in the pore solution. Discrepancies in totals arise from rounding (TS).
302
Cement Chemistry
varying proportions of inactive minerals (e.g. quartz. klspars, pyroxenes. magnetite) that are harmless. Others contain constituents. such as organic substances or clays, that interfere with setting or strength development. Zeolites or minerals of related types that are found in natural pozzolanns include analcinie, leucite, chabazite, phillipsite and clinoptilolite. The last of these occurs in substantial quantities virtually pure. Zeolites are readily synthesized by mild hydrothermal treatment of starting materials of appropriate composition, and their formation as alteration products of volcanic glasses during cooling is easily understandable. Diatomaceous earth is composed of the siliceous skeletons of microorganisms. I t is pozzolanic, but its use in concrete is much restricted by its very high specific surface area, which greatly increases the water demand. Some clays react significantly with lime at ordinary temperatures. but while this property can be of value for soil stabilization, their physical properties preclude their use in concrete. Many clay minerals yield poorly crystalline or an~orphousdecomposition products at 600-900•‹C (Section 3.3.2). and if the conditions of heat treatment are properly chosen, these have enhanced pozzolanic properties. Heat-treated clays, including crushed bricks or tiles, can thus be used as pozzolanas; in India, they are called surkhi. Other examples of natural rocks that have been used as pozzolunas. usunlly after heat treatment, include gaize (a siliceous rock containing clay minerals found in France) and moler (an impure diatomaceous earth from Denmark). The heat-treated materials are called 'artificial pozzolanas'. and this term is sometimes used more widely, to include pfa. Table 9.7 gives the chemical compositions of some natural pozzolanas. Many of the volcanic materials are not grossly different in composition from typical low-lime p h , which they also resemble in having a glass high in SiO, and A120, as a major active constituent. They differ in having zeolites as an additional active constituent and in their ~iiicrostructurc and physical properties. Some of the natural pozzolanas have high ignition losses, due wholly or partly to the presence of zeolites.
9.4.2
Hydration reactions
Early studies, reviewed by Malquori (MSI), showed that natural po~zolanas take up CH, including that produced by Portland cement. with the formation of products similar to those formed on hydration of the latter material. They also showed that the zeolites present in many of them were at least as reactive in this respect as the glassy constituents. Zeolites are cation exchangers, but the amounts of CaO they take up are much greater than can be thus explained; moreover, cation exchange could not explain the develop-
Table 9.7 Chemical compositions of some natural pozzolanas
Sacrofano (Italy) Bacoli (Italy) Segni (Italy) Santorin Earth Rhenish Trass Rhyolite pumice
Na20
MgO
AI20,
SiO,
K20
SO,
CaO
Fe,O,
-
-
1.2 4.42 2.0 1.20 1.23
3.05 18.20 19.59 13.0 18.29 15.89
89.22 53.08 45.47 63.8 52.12 65.74
-
3.08 0.85 3.8 1.48 4.97
2.28 7.61 0.16
0.77 9.05 9.27 4.0 4.94 3.35
4.67 4.29 9.91 5.7 5.81 2.54
From a compilation in Ref. M82
0.65 6.35 2.5
-
-
5.06
1.92
-
Loss -
3.05 4.03 4.8 11.10 3.43
Total 99.99 100.24 100.05 99.6 100.00 99.07
304
Cement Chemistry
ment of strength that occurs on mixing with lime and water. I t co~lldbe the initial process, but the main reaction is one in which the aluminosilicatc framework of the zeolite is destroyed and C-S--H and hydrated aluminate phases are formed. The broad similarity between the chemical and phase compositions of many natural pozzolanas and Pfa's suggests that one might expect to obtain similar products, though not necessarily the same rates of reaction. Striitlingite (C,ASH,; Section 6.1.4) was first synthesized by reaction between CH and metakaolin in suspensions at ordinary temperature ( Z 9 ) . Depending on the composition and conditions, similar reactions of natural pozzolanas can yield, besides C-S-H, striitlingite, hydrogarnet or C,AH,. Sersale and Orsini (S93) studied the reactions of herschelite [a synonym for chabazite; KNa(Si,A1,Ol,)~6H2O approx.], analcime [Na(AISi,O,).H,O approx.] and some natural pozzolanas, synthetic glasses and blastfurnace slag glasses with saturated CH solution in suspensions at ordinary temperature. In all cases, substantial amounts of CaO were removed from the solution, and XRD showed that C-S-H and one or more hydrated aluminate phases had been formed. In many cases, the presence of a 0.167-nm peak showed that the C-S-H was more specifically C-S-H(1). Except with the slag glasses, the aluminate phase most often formed in these reactions was stritlingite. When it was not found, C,AH, C,ACH , , or hydrogarnet was usually detected. Sersale and Orsini considered that high concentrations of SiO, and A1203, which might arise through the presence of alkalis, favoured the formation of striitlingite and that high concentrations of CaO and Al,O, favoured that of hydrogarnet; C,AH,, was formed from glasses high in SiO, and low in AI,O,. The reactions of the natural pozzolanas with CH in pastes were also examined. In all cases. the products were C-S-H, stritlingite and C,AH,, or C,ACH, ,. In agreement with earlier results, the natural and synthetic glasses, tuffs and pure zeolites were found to be of comparable reactivity. Costa and Massazza (C44) concluded from a study of natural pozzolanas of varied types that reactivity in mixtures with CH at w/s = 3 and 40 C depends during the first 28 days on the specific surface area and at later ages on the contents of SiO, and A1203in the active constituents. A comparative study of five natural pozzolanas and three low-CaO pfas in pastes with cement showed that the CH contents of the pozzolanic cements were considerably lower than those of the pfa cements at 3-60 days, but virtually the same at 90 days, the pozzolanas thus appearing to react more rapidly than the pfas at early ages but more slowly later. Determinations of the unreacted mineral admixture in pastes with CH showed that at 90 days 2330% of the natural pozzolana had reacted, compared with 1 1-150A for the pfas. The similarity in CH contents suggests, however, that these values may not apply to mixtures with cement.
,.
Composite Cements
305
X-ray microanalyses of C,S-pozzolana pastes showed a gradual decrease in Ca/Si ratio on passing from regions near the ~tnreactedC,S to ones near the pozzolana (021). TMS studies of pastes of C,S. P-C,S and cement with and without natural pozzolanas (U9,M44) showed that, in the presence of the latter, formation of polymeric anions is accelerated and their mean molecular weight is increased. TMS results and determinations of combined water showed that the hydration of P-C,S is almost completely suppressed in the presence of a pozzolana and that, in pastes with C,S, 16-29'/0 of the poz~olanahad reacted in 180 days (M44). The effect on P-CIS hydration is similar to that found using QXDA for pfa (D12). Chemical extraction showed 10-45% of the pozzolanas in pastes with C,S to have reacted in 38 days, compared with under 10% for a pfa (U9).
9.5 9.5.1
Microsilica (condensed silica fume) Properties
Microsilica is a byproduct of the production of silicon or silicon alloys by rzducing quartz in an electric furnace. Some SiO is lost as a gas and is oxidized by the air, giving a very finely particulate solid. The material may require some benficiation. The particles are spherical. typically around 100 nm in diameter, and consist largely of glass; Table 9.8 gives chemical compositions. The material formed in making elementary silicon typically contains 94-98% SiO,, but for that formed in making the alloys 8 6 9 0 % Si0, is more usual. Typically, the average particle size is about 100 nni and the N, specific surface area is 15-25 m2 g - ' (H53). The true density is above 2000 kg m-3 but the bulk density of the freshly filtered powder is only 200 kg m-,. This makes the material difficult to handle unless it is agglomerated into nodules or used as a slurry. A comparative study of microsilicas from 18 sources showed considerable variation in composition and properties, one of those examined containing as little as 23% of SiO, and having a specific surface area of only 7.5 m2 g - ' (A2I). The same study showed that in most of the samples the diffuse XRD peak from the glass accounted for 98-99.5% of the total diffracted intensity and that it peaked at the value of 0.405 nm characteristic of vitreous silica. The commonest crystalline impurities detected were KCI, quartz, metallic iron and iron silicide, and pozzolanic reactivity was found to depend more on the chemical composition and nature of impurities than on the fineness or SiO, content. A surface layer of carbon, if present, greatly decreased reactivity. Viewed as a mineral addition in concrete, microsilica is characterized by its small particle size and high pozzolanic activity. In the absence of a water-
306
Cement Chemistry
reducing admixture, the percentage replacement of cement is probably cfrectively limited by the high water demand to about 5%, but this can be increased by adding superplasticizers. By lowering the w/s ratio in such mixes to 0.2-0.3, concretes with high compressive strengths can be obtained (A22). Bleeding is also reduced. With special mixes and processing conditions, compressive strengths of up to 270 MPa have been obtained (Section I 1.9). Table 9.8 Chemical compositions of microsilica from the production ofclcluentary silicon and 75% ferrosilicon alloy (H53) 75 %
Fe-Si
S
Loss
9.5.2
Hydration reactions
Traetteberg (T47) showed that microsilica used as an addition with cement has considerable pozzolanic activity, mainly in the period 7-14 days after mixing, and that the reaction product formed with C H probably had a Ca/Si ratio of about 1.1. Several subsequent studies have shown that the pozzolanic reaction is detectable within hours and also that the early reaction of the alite is accelerated (H37,H54,H55). Huang and Feldman (H54,H55) studied the hydration reactions in some detail. In pastes with 10% or 30% replacement and w/s ratios of 0.25 or 0.45, the C H content passed through maxima usually within the first day before beginning to decrease; in those with 30% replacement, it had reached zero by 14 days. Table 9.9 gives some of the results obtained for C H content and non-evaporable water in these pastes. As with pfa cements, and for the same reason, the non-evaporable water contents of mature pastes are considerably lower than those of comparable pastes of pure Portland cements.
Table 9.9 Contents of calcium hydroxide and non-evaporable water in some pastes of Portland cement with and without microsilica (percentages on the ignited weight) (H53) Weight ratios in mix Cement
Microsilica
Water
Maximum in CH content
CH content at 180 days
YO
(%)
Age (days)
Non-evaporable water content at 180 days (%)
308
Cement Chemistry
X-ray microanalyses show that the Ca/Si ratio of the C-S~-H decreases markedly as the content of microsilica increases. For the product surroundi n the clinker grains in a mortar made with a cement containing 30% of crystalline slag. Regourd ct (11. (R26) found this ratio to be 1.70; for a cement containing 25% of slag and 5% of microsilica it was 1.43. Comparable decreases were observed for cements containing granulated slag or pfa. Ca Si ratios of 1.3 and 0.9 were reported for 200-day-old pastes with 13% and 28% replacements, respectively (R43). Uchikawa (U12) reported a value of 1.43 for Ca/Si. with substantial aluminium, 0.33% N a 2 0 and 0.32O/0 K 2 0 . for a paste with 10% replacement cured for 60 days at 35•‹C. There d o not appear to be any data on the quantities of microsilica consumed. but these may be roughly estimated from C H contents and C-SH compositions, using a method of calculation analogous to that outlined in Section 9.3.6 for low-calcium pfa. The following conclusions thus obtained are based on the data of Huang and Feldman (H55) for 200-day-old samples, and quantities are referred to 100 g ignited weight of composite cement. In the paste with 30 g of microsilica, in order to eliminate all the CH and bring the Ca/Si ratio of the C-S-H down to 0.9, about 22 g of microsilica would have to react. In the paste with I0 g of microsilica, to give a C H content of 8.3% and a Ca/Si ratio of 1.43, virtually all the microsilica would have to react. On the assumption that the Ca/Si ratio of the C----S-H does not fall below 0.8, the maximum amount of microsilica that can react is about 35g per 100g of cement, assuming that hydration of the latter is substantially complete.
9.6 Other mineral additions 9.6.1 Class C fly ash
Pfa formed on burning lignite or sub-bituminous coal is typically higher in CaO, MgO and SO,, and lower in SiO, and A1,0,, than the low-CaO materials described in Section 9.3. Bulk chemical compositions vary widely A1203), and thus normally (Table 9.10). Pfa with less than 70% (SiO, high in CaO, is termed Class C fly ash in the USA. As with pfa low in CaO, fine, spherical particles consisting largely of glass predominate, but the glasses are higher in C a O and the assemblages of crystalline phases are different. In a study of 26 ashes from the western USA. McCarthy et 01. (M84) found that quartz, lime, periclase, anhydrite and a ferrite spinel were ubiquitous or nearly so. Other crystalline phases present in some cases or in minor amounts included C,A, merwinite, alkali sulphates, melilites, mullite, sodalite and possibly others. Due to the presence of
+
Table 9.10
Chemical compositions of some fly ashes high in CaO (S94)
Origin
NatO
MgO
A1,0,
SiO,
SO,
K,O
CaO
Fe,O,
Loss
Total
Montana, USA North Dakota, USA North Dakota, USA Saskatchewan, Canada
0.2 7.3 5.4 6.1
6.8 7.9 5.5 2.9
20.3 12.5 10.7 21.9
35.2 30.2 27.9 47.9
1.1 9.6 12.3 1.1
0.5 0.6 1.5 1 .O
25.0 23.6 21.6 13.3
6.3 4.6 9.9 4.9
0.3 1.8 1.4 0.1
95.7 98.1 96.2 99.2
31 0
Cement Chemistry
free lime, some Class C fly ashes set and harden on being mixcd with water alone, and are thus true hydraulic cements. The combination of C,A and anhydrite, if present, contributes to the hydraulic activity by forming ettringite. In some cases, the state of the lime or the content of anhydrite is such that delayed expansion may occur. Diamond (D40) found a linear relation between the position of the glass peak in the XRD pattern and the content of CaO in the bulk analysis. This is consistent with observations on other glasses that the peak position moves to higher angles as the proportion of network modifiers increases, but the peak position is unlikely to depend only on the bulk composition because varying amounts of CaO are present in crystalline phases. In later work (D41), two fly ashes of broadly similar composition were compared. One, with 28.7% CaO, contained much lime and anhydrite, and gave a glass peak at 27" 20. The other, with 25.8% CaO, was low in these phases and gave a glass peak at 32" 20. The two ashes behaved differently on mixing with water, but these were less pronounced if cement was also present. The particles of coal burn essentially independently of each other, and are likely to vary both in the nature and relative amounts of their inorganic constituents and in heating and cooling regime; this is likely to produce wide variations in the bulk chemical and phase compositions of individual fly ash particles (D42). Hemmings and Berry (H56) reviewed work on fly ash glasses, including microstructural, spectroscopic, DTA and chemical evidence. Two types of glass, called I and 11, can be concentrated by density separation. Glass I contains minimal proportions of Ca" and other network modifiers, and tends to form relatively large, sometimes hollow, particles. Glass I1 contains substantial contents of network modifiers, and tends to form smaller, solid particles. The diffuse XRD peaks in general result from the overlap of contributions from these constituents. The mechanisms of formation of the particles were discussed. Evolution of H 2 0 and CO,, and the varying viscosities of liquids of different compositions. are important factors affecting particle size and cenosphere formation. Little has been reported on the hydration chemistry of high-CaO fly ashes. Diamond and Lopez-Flores (D39) found that at ages ranging from I h to 180 days, 30% replacements of a Portland cement by any of three difTerent high-CaO ashes produced only small decreases in the content of nonevaporable water. This contrasts with their observations with low-CaO fly ashes (Section 9.3.6). As was noted in Section 9.3.6, the non-evaporable water content depends mainly on the amount of CaO in the hydration products, and the observation thus suggests that hydration of the high-CaO ashes occurs at a rate comparable with that of the clinker phases. Grutzeck et al. (G71) examined some pastes made using a high-CaO fly ash and a coarsely ground Portland cement by XRD and SEM. The hydration
Composite Cements
31 1
products detected at ages of 3-56 days were essentially those given by pure Portland cements at similar ages, with the addition in all cases of striitlingite and, at 3 days, a trace of C2AH,. The SEM showed that the particles of tly ash glass were replaced in situ by radial growths of fibrous C - S - H . These were barely detectable at 3 days but reaction was seen to be well advanced by 7 days. The formation of C2AH, and stritlingite indicates substantial release of AI20, from the fly ash, even at 3 days.
9.6.2 Other pozzolanic or hydraulic additions
The ash from burning of rice husks or some other agricultural wastes is high in SiO, and if burning is carried out under properly controlled conditions at 350-600 C the ground material is strongly pozzolanic (M85,C45,J31). The material thus formed consists largely of amorphous silica retaining the cellular microstructure of the original material and has a very high specific surface area. Its possible utilization in conjunction with C H or cement has received attention for reasons of economics and energy conservation, and also because it can be used to make concrete that is very resistant to acid attack. Rice husk ash has characteristics broadly similar to those of microsilica but can be even more strongly pozzolanic. A study of pastes with C,S (K48) showed the C H content to pass through a maximum of 3 % at 7 days. referred to the ignited weight; by 28 days, it had fallen to I %. The hydration product was C-S-H with a Ca/Si ratio estimated at about 1.3 by analytical clectron microscopy, or 0.9--1.2 from the contents of CH and unreacted starting materials. There were indications that an initial product with a C a Si ratio of 0.1-0.2 was formed. The nature and availabilities of specific industrial waste products may be cxpected to vary with changes in technology, including those resulting from depletion of existing sources of energy o r raw materials or from increasing recognition of the need to reduce environmental damage and pollution. As one example, both SO, and NO, emissions from coal conlbustion may be decreased by burning at a low temperature in the presence of limestone ('primary flue gas desulphurization') (022). A second example is provided by the Lurgi or other processes in which gaseous or liquid fuels are made from coal or lignite, which will become increasingly important as resources of petroleum and natural gas are depleted (e.g. Ref. N17). In all these cases. residues are produced which, though of limited interest at present, could become major constituents of composite cements or concrete at some future date. A number of other waste materials, such as slags from production of steel or non-ferrous metals, are also of potential interest (R34).
31 2
Cement Chemistry
9.6.3 Calcium carbonate a n d other fillers Composite cements may contain mineral additions other than. or as well as, ones with pozzolanic o r latent hydraulic properties. Regourd (R34) reviewed the use of ground limestone, which is widely used in France in proportions of up to 27%. The limestones used consist substantially of calcite. with smaller proportions of quartz or anlorphous silica and sometimes of dolomite. They must be low in clay minerals and organic matter because of the effects these have on water demand and setting, respectively. The X R D peaks of the calcite are somewhat broadened, indicating either small crystallite size or disorder or both; I R spectra confirm the occurrence of disorder. The limestone is commonly interground with the clinker, and because of its softness becomes considerably more fine than the latter. For an overall specific surface area (Blaine) of 420 m2 kg- I , 50% of the filler can be below 700 nm, compared with 3 pm for the clinker. The effect on 28-day strength of partial replacement of clinker by filler can be compensated by finer grinding of the clinker, and for equal 28-day strengths, the I-day strength of a cement containing filler can be higher than that of one not containing it. This is due to the greater fineness of the clinker and the effects of the filler. The effects of the limestone are partly physical and partly chemical. As with many other finely divided admixtures, including pfa, the hydration of the alite and aluminate phases is accelerated. Because of its fineness the material also acts as a filler between the grains of clinker. though it is unlikely to be as effective in this respect as microsilica. Chemically, it reacts with the aluminate phase, producing C,ACH, thus competing with the gypsum. The increased rate of reaction of the alite in the presence of a limestone filler was demonstrated by light microscopic and SEM determinations of unreacted clinker, SEM observations on the thickness of the zones of hydration product surrounding the alite, and X R D observations on relative contents of C H ((372). In pastes with a high proportion of filler, the CH is concentrated into large masses, which form bridges between the grains of filler. This contrasts with the smaller and more evenly dispersed crystals formed in the absence of filler. The presence of up to 25% of fillers does not greatly affect workability, but with highly reactive or finely ground clinkers there can be some acceleration of set, probably due to formation of c,ACH,, (Bll6).
,,
9.7 P o r e s t r u c t u r e s a n d t h e i r r e l a t i o n to p h y s i c a l p r o p e r t i e s 9.7.1
Porosities a n d pore size distributions
Studies on the pore structures of pastes of composite cements have presented
Composite Cements
31 3
difficulties. which probably arise from discontinuity of the pores. There is also strong evidence that mercury intrusion alters the structure. Fcldman (F35) carried out experiments in which mercury was intruded, removed and reintruded into mature pastes. With Portland cements, the curves obtained on the second intrusion differed little from those obtained on the first. though the method cannot show whether damage occurs during the first intrusion, and there are indications that this is the case (Section 8.3.6). With slag or pfa cements the apparent pore structure became coarser. Fcldman concluded that the pastes of composite cements contained relatively large but discontinuous pores, separated by walls that were broken by the mercury. Broadly similar changes on repeated intrusion were observed ivith microsilica cements (F41).
ABSOLUTE PRESSURE
( M Pa)
Fig. 9.6 Pore size distributions for pastes o r Portland and coniposite cements (wls = 0.45. curing temperature 21•‹C).determined using mercury intrusion porosimetry. CI D. C I Y : Portland cement pastes cured for I day and I ycar. S2D. SI Y: Portland-blastfurnacc slag cement (70% slag) pastes, cured for 2 days and 1 year. F3D. FIY: pfa cement pastes (13% CaO in fly ash) cured for 3 days and 1 ycar. After Feldman (F34).
Fig. 9.6 compares curves obtained on first intrusion for pastes of Portland and composite cements. At early ages, pfa o r slag cement pastes are more porous than comparable Portland cement pastes because of the relatively slow reactions of the mineral additions, but the pore size distribution curves indicated by MTP are essentially similar in shape. For mature pastes. the apparent distributions for the composite cements indicate that there is a greater proportion of fine porosity. For the pfa cement paste, the mercury porosity obtained a t maximum pressure are greater than that of the Portland cement paste. Day and Marsh (D32) agreed with Feldman that discontinuity
31 4
Cement Chemistry
in the pore structure produced regions that were inaccessible to mercury until the pressure was high enough for the mercury to break through the blockages, and that an inaccurate assessment of the pore size distribution resulted. For Portland cement pastes, mercury porosities agree with those obtained using helium except at low porosities, when they are lower (BI 10). For pastes containing pfa or silica fume, they tend to be higher than those obtained with either helium or alcohols (F41,M86,D32). presumably because the latter fluids cannot break into the blocked regions. For pastes containing microsilica, they are lower than those obtained by equilibration with water at 11% R H (F41). This indicates that even mercury at high pressure does not completely enter all the pore space. Feldman and Huang (F41) concluded that in blends with microsilica a discontinuous pore structure was formed within 7 days. Uchikawa et 01. (U18) studied pastes of Portland and composite cements by N, sorption. In all cases, the pore size distributions peaked at 2 nm. Partial replacement of cement by slag increased the height of the peak. and partial replacement by pfa decreased it. Calculations based on reaction stoichiometry and densities of phases support the conclusions from experimental observations that mature pastes of composite cements are more porous than comparable pastes of Portland cements. This is indicated by the results in Table 7.3. 9.4 and 9.6. Similar calculations for 180-day-old pastes of w/s 0.45 indicate free n.ater porosities of about 24% for a typical Portland cement, 35% for a cement with 40% slag, 35% for one with 40% pfa and 32% for one with 30% microsilica. The calculated values are in all cases somewhat higher than observed mercury porosities (F34,F4 1).
9.7.2
Relations between pore structure and physical properties
Feldman (F34) plotted the logarithms of compressive strength and Young's modulus against mercury porosity for a number of pastes. and extrapolated to zero porosity. For both quantities, the intercept at zero porosity was higher for pfa cements than for Portland cements. Slag cements also gave a higher intercept than Portland cements for Young's modulus, but the intercepts for strength were virtually identical. While the precise significance of the mercury porosities is uncertain, the results suggest that pastes of composite cements are less deformable for a given porosity than those of Portland cements and that pfa cement pastes are also stronger. Feldman considered that this might be because the pastes of composite cements contained more C-S-H and less CH.
Composite Cements
31 5
Partial replacement of Portland cement by a mineral addition can greatly decrease the permeability to water provided the age is such that sufic~ent reaction of the addition has occurred. For a paste with w/s 0.47 and 30% replacement of cement by pfa, cured at 20•‹Cfor I year, Marsh ct d.(M87) found a permeability of 10-'5.4m S-'; the corresponding paste of pure Portland cement had a permeability of 10-".'m s - ' . For the pfa cement pastes, the decreases in permeability were found to run parallel to the consumption of C H in the pozzolanic reaction. The permeability is similarly decreased through the use of microsilica (H47), slag (R44,M86) or rice husk ash (M86). Its value has been correlated with pore size distribution (M86). but it is more likely to be related to the continuity of the pores (M87).
Calcium aluminate, expansive and other cements
10.1 Calcium aluminate cements 10.1 .I
Introduction
A cement niade by fusing a mixture of limestone and bauxite or other aluminous material low in silica was patented in 1908 by Bied, of the Pavin de Lafarge Company in France. I t was developed as the successful result of a search to find a cement having high resistance to chemical attack by sulphates, and became known as Ciment Fondu. Calcium aluniinate cements were found to have excellent resistance to many other forms of chemical attack and to set normally but to harden rapidly: concrete can be produced that has useful strength in 6 h and a strength at 24 h equal to that given by Portland cement in 28 days. They were also found to pro\ ide good abrasion resistance in low-temperature applications. I t was found that their successful use in a wide range of general and specialized applications depended on proper mix design and workmanship. and that failure to meet these requirements could lead to deterioration of the concrete. The total quantity of heat evolved by a calcium aluniinate cement on hydration is typically 325-400 kJ kg-', which is somewhat less than that for a typical Portland cement, but due to the speed of the reaction. nearly all of it is produced during the first day. Calcium aluminate cements arc therefore suitable for use at low temperatures (down to - 10•‹C). e.g. for winter construction in cold climates or repairs in cold stores. Mixtures of calcium aluminate and Portland cements, sonietinies also with slaked lime (CH) or additional gypsum, can be used to make rapid setting mixtures for grouting and similar applications. The strengths obtainable (e.g. 25 MPa at 5 h) ((373) are relatively high for cements of this type. Used with appropriate refractory aggregates, calcium aluminate cements may be used to make refractory castables having applications in the steel and other heat-using industries. Cements niade from bauxite ma!. thus be used at hot face temperatures up to 1350•‹C. White calcium aluminate cements, which are low in iron and higher in alumina, can be used with appropriate aggregates to make castables similarly resistant up to 2000•‹C.
Calcium Aluminate, Expansive and Other Cements
31 7
Refractory custables have good resistance to thermal shock. Calcium aluniinate cements have also been termed aluminous or highalumina cements, but the former term is less specific and the latter is more appropriately used for the white cements high in AI,O, mentioned above. The cements described here are those manufactured in France and the U K . Those made elsewhere are similar in essential respects.
10.1.2 Manufacture; chemical and mineralogical compositions
Cirnent Fondu is normally made by complete fusion of limestone and bauxite at 1450--1600•‹C. In order to produce a cement with the desired rapid-hardening properties, both raw materials must be low in SiO?. The molten clinker is tapped off continuously from the furnace. solidifies and is typically crushed and ground to a fineness of about 300 m2 kg-'. Some iron is reduced to F e z + . The colour of cements produced from bauxite can vary from yellow brown to black, but is commonly greyish black. White calcium aluminate cements are usually made by sintering calcined alumina with quicklime (calcium oxide) o r high-purity limestone. Table 10.1 Typical compositions of calcium aluminatc cements (weight pcrccntagcs)
+
Type of cement AI,O, CaO Citnent Fondu 40"h Alumina 50% Alumina 50% A1,0, (LOWFc) 70% Alumina 80% Alumina
FcO Fe,O,
K20+ FeO SiO, TiO, MgO Na,O
3 8 4 0 37-39 15-18 3-6 4 0 4 5 4 2 - 4 8 < 10 <5 49-55 34-39 <3.5 < 1.5 50-55 36-38 < 2 69-72 27-29 <0.3 79-82 17-20 <0.25
3-5 5-8 4-6
2 4 < 1.5 <0.4 2 2 < 1.5 <0.4 22 1 <0.4
--
SO, <0.2 <02 <0.3
2 2 -1 <0.4 <0.3 <0.1 <0.3 ~ 0 . 5 <0.3 ~ 0 . 1~ 0 . 2~ 0 . 7 <0.7
Table 10.1 gives typical chemical compositions of commercially produced calcium aluminate cements. The essential compound in all of them. because it develops the main hydraulic activity and is consequently responsible for the strength development, is monocalcium aluminate, CA. In white calcium aluminate cements, it can occur with various combinations of the other binary phases of the CaO-A120, system and corundum. The crystal structures of these compounds and the phase equilibria relating to their formation were considered in Chapter 2. In the sintering process by which these cements are made, the reaction conditions are of the utmost importance, as
31 8
Cement Chemistry
the products of a given mix can vary from a mixture of CA and CA, to the full range of phases CaO-C,A-C,,A,-CA-CAz-CA6-AI,O, (S95). Most of the commercial cements contain CA and C,,A7; those high in AI,O, may also contain CA,, CA, or a-Al,O,. In Ciment Fondu, most of the grains of the ground cement observable by backscattered imaging in the SEM are polymineralic (H57). Sorrentino and Glasser (S96) discussed qualitative and quantitative phase con~positions. The crystalline phases may include five or even six of the following: CA, ferrite of one or more compositions, C,,A,, P-C,S, C2AS. pleochroite and wustite. Perovskite (CaTiO,) may also be present, and glass if crystallization is incomplete. CA typically makes up 40-5O0/0 of the material, and is a solid solution often containing Fe3+ and a little silicon; the liniit for Fe3+ is probably about 5% expressed as Fe,O, (532). The j2witc phase [Ca,(AI,Fe, _,),O,; Chapter I] constitutes 20-40%. The composition, which includes up to 3.5% of TiO,, varies more widely than in Portland cement and depends on the manufacturing conditions (S97). X-ray microanalysis is easier than in Portland cement clinkers because there is no intergrowth with aluminate phase. The other phases are present in amounts generally below 10% and often below 5% (S96). The C,S (Section 1.3) contains iron and aluminiuni in similar proportions to the belites of Portland cement clinkers. The grhleni/c (C,AS; Section 2.2.5) and C1,A, typically contain about 9% and 50h of Fe,03, respectively (S97). The behaviour of the melilite during the crystallization of the Fondu melt can be followed on the pseudo-binary phase diagram C2AS-'C2FS1, where 'C,FS' is a mixture of phases (S98). Pl~oc,hroitc, of possible composition C ~ , , A ~ , , , , F ~ ~ ~ ~ M and ~ , ,the~ F ~ ~ ~ ~ S ~ ~ ~ ~ O ~ ~ . calcium-containing wustite [(Fe,Ca)O] were described In Chapter 2. Neither the C,S nor the wiistite is easily detectable by XRD. Sorrentino and Glasser (S98) descr~bed phase equilibria in the CaO-AI,O,-Fe203-SiO, system. For compositions similar to those in Ciment Fondu under oxidizing conditions, the possible equilibrium assemblages of four solid phases are CA-C2S-C,AS-ferrite and CA-C,S-C,,A7-ferrite, where CA, C,AS and ferrite represent solid solutions. Under mildly or moderately reducing conditions (S99), either pleochroite or wustite or both may also be present, the possible equilibrium assemblages of five solid phases being CA and ferrite together with (a) C,S, C12A7and pleochroite, (b) C,S, C,AS and pleochroite or (c) C,AS, pleochroite and wiistite. These results imply that ferrite is always likely to be present and that Cl,A7 cannot coexist stably with C,AS. Caution is needed in applying them to Ciment Fondu, because equilibrium is not necessarily reached, so that phase assemblages incompatible at equilibrium may occur. Because of the complexity and variety of the possible phase assemblages, the wide ranges of solid solutions and departures from equilibrium arising
Calcium Aluminate, Expansive and Other Cen~ents 31 9
from the nature of the tnan~tfact~~t-ing process, the calculation of quantitative phase composition from bulk chemical analysis presents a much more difticult problem than for a Portland cement. Calleja (C46) obtained approximate solutions by omitting the FeO and other minor components and making suitable assumptions as to the compositions of the CA. C,,A- and C,AS phases. Sorrentino and Glnsser (S96) also considered the effects of FeO and MgO and consequent formation of pleochroite and wiistite.
10.1.3 Reactivities of the phases and methods of studying hydration
In commercial calcium aluminate cements, the only phases that hydrate significantly at early ages are nornlally CA and, if present. C , ,A-. Some sintered cements made at relatively low temperatures also contain free lime or CH. which are very active in the hydration process. The reaction of CA2 present in many white cements high in AI,O,, is very slow (B1 17). possibly because of gel formation. In Ciment Fondu, the ferrite phase seems to play no significant part in early hydration at 20•‹C.but at 3038•‹Cover 80% was found to have reacted by 2 months (C47). The melilite and pleochroite seem to be unreactive. When belite is present, silicate ions can be detected in the solution \iithin it few minutes, but then disappear; it seems that precipitation occurs and further dissolution is inhibited. Among the minor oxide components. TiO, and MgO mainly occur in the unreactive phases. NazO and K,O scarcely affect the solution equilibria at early ages, as their concentrations are \ w y low (M88). The reactions of the anhydrous phases with water were first studied by LeChatelier, whose conclusion that they occur by congruent dissolution followed by precipitation of hydration products has been supported by many subsequent investigations, which have established the reaction mechanisms in greater detail and the nature of the products. The methods that have given the most information have been qualitative or quantitatiw XRD. determinations of solution compositions, thermal methods, conduction calorimetry and studies of setting behaviour. Among recent studies may be noted those of Barret and Bertrandie (BI 18,B119),Cottin and George (C47), Rodger and Double (R45), Fujii et al. (F42), Menetrier-Sorrentino cJr (11. (M89). Edmonds and Majumdar (E5,E6) and Capmas and MenetrierSorrentino (C48). Other methods have included neutron diffraction (C49), NMR (M90,R46), TEM (C50), SEM (H57), IR (B120,P45) and Miissbauer (H58) spectroscopy, determinations of electrical conductivity (G74.M88) or dielectric constant (A23), and ultrasonic wave propagation (G75). These methods have been applied, according to their various characteristics. to mortars, pastes and suspensions.
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Cement Chemistry
10.1.4 Hydration reactions and products The individual hydration products and relevant phase equil~bria were described in Chapter 6. The m~ijor initial products are CAH,, at low temperatures, C,AH, and AH, at intermediate temperatures and C,AH, and AH, at higher temperatures (Fig. 10.1). With the passage of time. the other products are replaced by C,AH, and AH, at rates that depend on the temperature and other factors; this process is called conversion. An amorphous or gel phase containing C a Z +has been reported to form simultaneously w ~ t hthe crystalline products, in a proportion relative to the latter that increases with temperature (E5,E6). 12 10
I
I
I
20
30
-
a6
-
4
-
CE-MENT
-
CONCRETE
0
10
TEMPERATURE
40
(OC)
Fig. 10.1 Major hydration products and typical setting times for neat pastes and concretes of Cimenl Fondu cured at various temperatures. George (G73).
Fig. 10.2 shows the changes in the solution composition that occur when a typical Ciment Fondu reacts with water a t 20•‹C, together with the metastable solubility curves for phases in the CaO-A1203-H,O system. The CaO and A1203concentrations increase, at an approximately constant CaO/ AI,O, ratio somewhat greater than 1; in the experiment shown, C,, reached 15 mmol 1 - in 3 min and its maximum value of 21 mmol I in I h.
'
Calcium Alummate, Expansive and Other Cements
321
Fig. 10.2 The Ca0-A1,0,-H20 system a t 20•‹C, showing calculated solubility curves and typical path (broken and dotted lines) followed by the composition of the solution obtained o n reaction of Cimcnt Fondu with water in a suspension with w/s = 10. AH, = poorly crystalline hydrous alumina; CA,, = superficially hydroxylated C A (see text). Capmas (C51) and Menetrier-Sorrentino (M88).
A solid product is thus being precipitated, and has a bulk CaO/AI,O, ratio lower than 1. It has widely been regarded as alumina gel, but there a r e several indications that minor amounts of C,AH, are also precipitated. * SEM examination shows the presence of foils, which could well be of poorly crystalline C,AH, (B121,M88) (Fig. 10.3). With some specimens of Ciment Fondu, the ascending curve shows minor arrests, during which Cca0 decreases (C48,M88); two such are shown in Fig. 10.2, near Cca, = 20 mol 1 -'. These arrests are also manifested by temporary reversals in the generally increasing electrical conductivity (G74,M88). Determinations of bound water show, however, that the total amount of hydrated material is are reached (B121,M88). very small until the maxima in Cca0 and CA1203 The subsequent course of the reaction, essentially as described by Capmas and Menetrier-Sorrentino (C48), is as follows. An induction period occurs, during which the concentrations remain near their maximum values, the
322
Cement Chem~stry
rates of dissolution and precipitation thus being approxinlately equal. The products nucleate and grow; these processes begin slowly. but accelerate. Ultimately, precipitation is massive, the concentrations drop and. in a paste, setting occurs. In the experiment shown, in Fig. 10.2, the concentrations began to decrease rapidly a t about 2 h and the final point was recorded at 5 h.
Fig. 10.3 SEM secondary electron image of Ciment Fondu hydrated for a few minutes in a suspension of w/c = 10 at room temperature. showing foils, probably of C,AH,. and amorphous material. Mknktricr-Sorrentino (M88).
Within minutes after mixing, the solution is supersaturated in AH,, CAH,,, C2AH8 and C3AH6. As noted earlier, AH, and C2AH, begin to form while the concentrations are still rising. For a cement in which CA is the only active constituent, CAW,, subsequently becomes a more important product below about 15•‹C (Fig. 10.1). For cements containing C,,A,, C2AH8 is a major product a t lower temperatures. In the conversion reaction, CAH,, and C,AH, redissolve and C,AH, is formed, together with additional AH,, the structure of which then approxi/,,mates to that of gibbsite. At 5"C, this process may take many years, but at \5O0C or above, the formation of C3AH6 and gibbsite is virtually immediate. The only phases in the pure CaO-A120,-H20 system stable in contact with aqueous solutions up to well above 100•‹Care gibbsite, C,AH, and CH.
-
Calcium Aluminate, Expans~veand Other Cements
323
10.1.5 Thermodynamic calculations When the anhydrous phases dissolve, the species present in solullon in significant concentrations are Ca2', AI(OH),- and O H - ; CaOH' may be ignored. Taking C2AH, as an example, the equilibrium with solution 1s thus represented by the equation
The solubility product is equal to { C a 2 + I 2x (AI(OH),-j2 x {OH;'. where curly brackets denote species activities and [OH-] may be replaced by 2[Ca2+]- [AI(OH),-1. As the concentrations are low, activity coefficients may be calculated from simplified Debye-Hiickel theory. Solubility products may thus be obtained from experimental data (N l8,Bl 18.B 1 19.C4X) (Table 10.2). The variations in solubility products with temperature may be represented by empirical equations of the form
where T is in "C (C48). and values of the constants u and h are included in Table 10.2. Standard free energies of formation may also be calculated ( N IS), but calculations of solubility products from them are numerically unreliable (C48). From the solubility products, again using activity coefficients calculated using simplified Debye-Hiickel theory, one may obtain calculated solubility curves (BI 18,B119,C48), some examples of which are shown in Figs 10.2 and 10.4. It is thus possible to calculate entire curves. such as that for CAH,, at 40•‹C, or portions of curves, such as those for C2AH, and C,AH6 at high AI20, concentrations, that are difficult or impossible to determine by experiment. Degrees of supersaturation relative to specific hydrates may also be calculated. The solubility curve of CAH,, is more markedly dependent on temperature than are those of C2AH, or C,AH6. At 5 • ‹ C a solution obtained from CA rapidly becomes highly supersaturated in CAH,,, but l n ~ ~ cless h so in C2AH, or AH,, but with increase in temperature this situation changes; thus CAH,, is the major product a t 5"C, but is undetectable above 30•‹C (C48). At 50•‹C, supersaturation is high only in C,AH,. which is rapidly formed. Barret and Bertrandie (BI 18,Bl 19,B124) explained the maximum concentrations reached in the CA-water reaction and the path by which they were attained. On contact with water, CA was assumed to develop a hydroxylated surface layer, termed CA,,, of composition Ca[AI(OH),],, which dissolves congruently and is continuously regenerated as new areas of surface are
324
Cement Chemistry
exposed. T h e maximum values reached by the C a O a n d A I z 0 , concentrations were considered t o represent the solubility of CA,,, from which a solubility product of 6.58 x mo13 k g - , was calculated. A solubility curve was thus obtained for CA,,, a n d is included in Fig. 10.2. Table 10.2 Equilibrium constants in the CaO-A120,-C0,-HzO
Reaction ( H 2 0 molecules in solution omitted) I. 2. 3. 4. 5. 6. 7. 8. 9. 10.
-=
AI, AH, (gibbsite) AH, (amorph.)+OHAI(OH),CAH,, Ca2 + 2AI(OH),s 2Ca2 + 2AI(OH),C2AH8 3Cazf + 2AI(OH),C,AH6 C,AH,, 4Ca2+ + 2AI(OH),CaCO, (calcite) Ca2+ C4ACHll 4Ca2 + 2AI(OH), CaOH CaZ' + O H AI,+ + 4 0 ~ AI(OH), 2
= = = = = =
+
+
+
2
system" log K
Ref.
+ 30H-
-33.5 +O.Z - 7.6 + 20H- 13.8 - 22.3 + 40H + 60H-25.2 C 0 , 2 - - 8.4 + 40H- +CoJ2- 2 9 . 3 + 1.3 32.4
+
+
Reaction
" 25-C and ~ o n i cstrength (I) = 0 except for No. 10, which is for I = 0.1 and 20•‹C Recalculated to give K as log to base 10.
Fig. 10.4 Calculated solubility curves for (A) poorly crystalline AH,, ( I ) CAH,,, (2) C2AH8 and (3) C,AH6 in the CaO-A120,-H,O system at 5"C, 20•‹Cand 40•‹C. Capmas (C5 1).
Calcium Aluminate, Expansive and Other Cements
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.
The path of increasing concentrations was shown to pass along a narrow corridor on the Cc,o-CAl2 3 diagram, enclosing compositions for which neither AH, nor C2AH, is rapidly precipitated. These phases were immediately precipitated on passing beyond the low-CaO and high-CaO boundaries of the corridor, respectively. The degree of supersaturation with respect to each of the hydrated phases was calculated for various combinations of C,,, and CAI03, and free energy changes for precipitation were thereby obtained. The which was termed a curve of minimum instability, was shown to correspond to a series of compositions for which the conflicting tendencies to precipitate AH, and C2AH, were as far as possible minimized. The hydration of C l 2 A , was considered in a similar way.
10.1.6 Setting times; mixing and placing
The setting times of most calcium aluminate cements pass through a maximum at 25-30•‹C (Fig. 10.1). The effect has been observed with Ciment Fondu and with white calcium aluminate cements, and thus does not depend on the presence of phases other than CA (8124). The setting time usually shortens between 0•‹Cand about 20•‹C, but at 28-30•‹C it can be up to eight times as long as at 20•‹C. Above 30•‹C, it again shortens very rapidly. The origin of the maximum is not clearly understood. It has been attributed to difficult nucleation of C2AH, (R45,B125), but may be rather associated with . , '.. that of CAH,, (C51). The length of the induction period decreases with the C/A ratio in the solution, which is in turn affected by the presence of C,,A, (BI 18). For ratios up to 1.06, given by pure CA, it can be as long as 6-12 h, but at ratios above 1.20, immediate setting can occur. The mechanism and products of hydration are essentially similar to those for CA, but, as might be expected from the higher CIA ratios in the solution, only a little CAH,, is formed. even at 4•‹C(E6). Increase in energy or time of mixing markedly accelerates the hydration of calcium aluminate cements, and if excessive can even cause quick setting in the mixer. Once mixing stops, progressive thickening occurs. Samples of a given paste mixed for different times can behave very differently during placing. This factor is often forgotten, and can lead to the addition of more water during placing, with a consequent increase in porosity and decrease in strength. The rheological properties of the fresh pastes (B126,C52) and potentials of suspensions ( Z l l ) are broadly similar to those found for Portland cements (Section 1 1.4.4). As with the latter, the water demand can be decreased by adjusting the particle size distribution. Factors affecting choice of w/c ratio are discussed in Section 10.1.8. -,,
c-
"
326
Cement Chemistry
10.1.7 Microstructural development Fig. 10.5 shows portions of fracture surfaces of some pastes of Ciment Fondu examined by secondary electron imaging in the SEM. At 22 11. in pastes hydrated at 20"C, fibrous material is abundant, together with plates of C,AH,. The fibrous material is probably partially dehydrated CAH Cottin (C50) earlier studied pastes of Cinient Fondu, largely by the TEM replica method. The microstructures of pastes of low w/c ratios hydrated for 7 days at 12•‹Cwere too compact to show more than occasional detail. but with pastes of w/c = 0.7 much detail could be observed in cavities exposed by fracture, which were lined with small crystals of CAH,, and sometimes contained plates of C2AH,. The crystals of CAH,,, which were hexagonal prisms up to 1 pm long and 0.2 pm wide, were present in rounded aggregates. Hydrous alumina was seen in a paste of w/c = 1.0 hydrated at 30-C; it occurred in the lining of a cavity, mainly as rounded masses of randomly oriented platelets. Hake and Pratt (H57) reported SEM observations on pastes hydrated at various temperatures. In those hydrated at 8•‹Cor 23•‹C. the main feature was fibrous material that was considered to be hydrous alumina, but ~ . h i c h could also have been partly dehydrated CAH,,. The hydrating grains of cement were surrounded by shells of hydration products, from which they tended to become separated in a manner similar to that observed with Portland cement pastes (Section 7.4.2) though the authors recognized that this could have been partly due to dehydration. Two-day-old pastes hydrated at 40•‹Cshowed spheroidal particles of C,AH, and thin, flaky plates of gibbsite. In pastes mixed with sea water, hydration took place more slowly, but no other effects on microstructural development were observed.
,.
10.1.8 Hardening; effects of conversion Calcium aluminate cements harden rapidly as soon as the massive precipitation of hydrates begins. This may be attributed to the fact that. unlike those of Portland cement, the major hydration products are cryst a 11'me. Relatively high proportions of water are taken up in the hydration reactions, the theoretical w/c ratios needed for complete hydration of CA being 1.14, 0.63 and 0.46 for the formation of CAH,,, C,AH, + AH, and C,AH, ?AH,, respectively. For this reason, and also because of the rapid heat
+
Fig. 10.5 SEM secondary electron images of the fracture surface of a paste of Ciment Fondu (w/c = 0.4), hydrated for 22 h at room temperature. showing (A) CAH,,. and (B) plates of C,AH,. Menktrier-Sorrentino (M88).
Calcium Aluminate, Expansive and Other Cements
327
328
Cement Chemistry
evolution, care is needed to avoid surface drying and even destruction of the concrete through the development of high internal vapour pressure. In blocks of concrete more than 300 mm thick, the temperature rise may be sufficient to cause rapid formation of C3AH6 during hardening under normal conditions. Strong bonds are formed with limestone aggregates (F43,B127), apparently due to formation of c,ACH,,, and with synthetic aggregates high in AI,O,, which give high abrasion and corrosion resistance (G76). The rate of conversion at a given temperature is reduced if the RH falls below saturation. The temperatures during setting and curing are also of major importance; if they are high, subsequent conversion occurs more rapidly, even though the temperature is then low (C47). This is probably due to the presence of nuclei of C3AH6. Conversion causes a decrease in the solid volume and thereby increases the porosity of the concrete. which results in at least a temporary decrease in strength. Calculation based on the densities of the hydrated phases (CAH,,, 1730kgrn-,; C2AH,, 1950kgrn-,; C3AH6, 2530kgm-3; AH,, 2440 kgm-3) shows that. for pure CA, the total volume of the hydration products decreases to 47% of its original value if the initial product is CAH,, and to 75% if it is a mixture of C2AH, and AH,. The importance of this effect depends on the w/c ratio. At a low w/c ratio, the space available is insufficient to allow all the CA to hydrate to CAH,, or to C2AH, and AH,. When conversion occurs. much water is released, so that even if the pores contain no water. further hydration can occur. In addition to the aluminate hydrates, C2ASH, may be formed (M91). At a high w/c ratio, more of the cement can hydrate initially, and the effects of conversion on the porosity and strength are correspondingly greater; carbonation also occurs more readily. These effects are illustrated by the curves in Fig. 10.6. The strength after conversion is affected by the rate at which that process occurs, probably because of the effect on C,AH6 morphology (M91). Strength may also be affected by crystallization of AH,, which could reduce both its pore-filling capacity and its efficacy as a binder. The size of the C3AH6 crystals increases with the temperature at which they are formed; this also may decrease strength. It has sometimes been said that C,AH, is an intrinsically weak binder, but this is not correct. As with relatively dense, crystalline substances in general (F36), it gives lower strengths than less dense substances at high porosities, but higher ones at low porosities. This may be seen from the crossing of the curves in Fig. 10.6. Hot pressed pastes of calcium aluminate cements, in which the binder is C3AH6 and the porosity has been reduced to about 4%, show strengths of up to 480 MPa (Section 1 1.9.1).
Calc~urnAluminate. Expansive and Other Cements
329
CALC.
x Z W
CL
-
v, /
CL
a
z
0
0
0.4 WIC
Fig. 10.6 Compressive strengths and observed and calculated porosities for pastes of Ciment Fondu of varying w/c ratios. The unconverted paste, which contained CAH,,, was cured for 7 days at 10•‹C.The converted paste, which contained C,AH, and AH,, was cured for 7 days at 70•‹C. After George (G73).
Conversion and its practical implications have been widely studied but not always well understood. Thus, concretes made with inadequate cement contents and excessive w/c ratios have failed in structural applications. An extensive investigation in the UK into calcium aluminate concrete in existing buildings (B128) showed that a high level of conversion was generally reached in a few years, causing a very variable loss of strength, but that this loss was in the majority of cases insufficient to endanger the structure. No adverse effects of corrosion on steel tendons removed from prestressed
330
Cement Chemistry
concrete beams in normal buildings were detected. The i ~ s eof calcium aluminate cements in structural work is nevertheless not deemed to satisfy the current (1989) U K Building Regulations. In France. prohibitive restrictions were for a time placed on its use in the public sector of the construction industry, but in 1968, a new set of rules was issued after lengthy studies. Current French regulations (M92) specify that the total wsc ratio (i.e. that including any water sorbed by the aggregate) must not exceed 0.4 and that the cement content must be at least 400 kg m - 3 . The evidence supports the view (B128,G76) that design should be based on the assumption that conversion will occur, tests using accelerated curing being carried out to determine the converted strength. Fig. 10.7 shows the results of some long-term tests; for design purposes. a rapid increase levelling off at the minimum converted strength should be assumed.
Fig. 10.7 Long-term compressive strengths of Ciment Fondu concretes cured for 24 h in moist air at 18•‹Cand then stored outdoors in southern France. After George ((376).
Conversion is of only minor importance for the use of calcium aluminate cements in refractory applications and in mixes with Portland cement such as those described in Section 10.1.10. 10.1.9
Chemical admixtures
Many inorganic salts affect the setting times of calcium aluminate cements and are effective in concentrations of 0.5% (L6,C53,R45). The effects on setting times differ greatly both in direction and in relative magnitudes from
Calcium Aluminate, Expansive and Other Cements
331
hose observed with Portland cements (Section 11.5.1). The results of different investigators show apparent discrepancies, and the e f i c t s probably depend greatly on concentration, which may even reverse their directton. but there is general agreement that lithium salts and alkali or calcium hydroxides are strong accelerators and that many common inorganic salts retard. As with Portland cement, hydroxylic organic compounds, such as sugars or citric. tartaric or gluconic acids, are powerful retarders. Rodger and Double (K45) studied the effects of several adnirxturcs, particularly LiCl and citric acid, by XRD, conduction calorimetry and solution analyses. The samples were slurries of a Ciment Fondu hydrated at 12 C and in the absence of admixtures gave CAH as the main product. In the mixes with LiCI, C2AH, was the main product and a little Li,O. 2AI,O,.l l H 2 0 was detected. The authors considered that this compound acted as a nucleation substrate for C,AH,. The very low concentrations in which the organic compounds are effective suggests that, as with Portland cement, they act by poisoning the nuclei of the hydration products. but the authors found this hypothesis incompatible with the observed solution composi tions. An experimental study of the CaO-A120,-H,O system in the presence of NaOH showed that at a concentration of I mmol I ' the latter had negligible effect on the equilibria, but that at 10mmol I - ' the solubility curves were displaced to lower C a O and higher AI,O, concentrations. At 100 mmol 1 -', the solutions were very low in CaO, and precipitation did not occur within 28 days (P46). The effect of O H - ion on the solubility curves can also be calculated by an extension of the procedure mentioned in Section 10.1.5 (B119). The results showed large shifts in the curves a t an NaOH concentration of 40mmol I - ' , the minimum in C a O concentration on the C2AH, curve at 21•‹Cbeing shifted from C,,, = 6.8 mmol kg-'. CA,,o, = 2.3 mmol k g ' to C,,, = 1 . I mmol kg- ', CAlZ03E 13 mmol kg-'. An experimental study on the effects of adding sodium phosphates of various types has been reported (B 129). Other types of admixtures used with calcium aluminate cements include water reducers and superplasticizers (Section 11.4), which also act as retarders, and thickening agents, such as carboxymethylcellulose. Complex formulations may be used for special purposes; for example, a ready-mix mortar for high-performance road repair might contain 55% Ciment Fondu and 4.5% sand, with glass fibre, aluminium powder, Li,C03. sodium gluconate and methyl ethyl cellulose (M93).
,
10.1.10 Mixtures with calcite, slag, gypsum or Portland cement Cussino and Negro (C54) reported that loss of strength in calcium aluminate
332
Cement Chemistry
cement mortars could be prevented by using calcareous aggregates or by adding finely ground calcite to the cement. They showed that C,ACH, was formed and concluded that this limited or prevented conversion: Fentinian (F44) confirmed the formation of C,ACH,, but found that at 75 C or below CAH was still the main product and that conversion was delayed but not prevented. C,ACH,, seems to form at the expense of C,AH, and C,AH, at 25 -60 C but to be unstable at o r above 60•‹C (M94,F44). Strong concentration gradients and other local inhoniogeneities (8130) may explain some of these apparently contradictory results. Equilibria involving C,ACH, have also been approached theoretically (B 122,B 13 1 ,S 100). Barret ct (11. (B122,BI31) concluded that CaCO, would dissolve giving C,ACH,, only at partial CO, pressures above a certain value, but that equilibrium with atmospheric CO, would not be reached within the pores of a concrete. (11. (M95) studied concretes containing equal weights of Majumdar Ciment Fondu and ggbfs. On water curing at either 20•‹C or 38•‹C. the compressive strengths continuously increased over a period of I year. whereas those of control samples made without the slag showed the normal decreases associated with conversion. Parallel studies on pastes showed that the superior behaviour of the mixes with slag were associated with formation of strstlingite.
,
,,
,
CIMENT FONDU
(Oh)
Fig. 10.8 Setting times of neat pastes of mixtures of Ciment Fondu and Portland cement. These are only general indications, as the times depend on Portland cement composition, mixing time, temperature and other factors. After Robson (R47).
Calcium Aluminate, Expansive and Other Cements
333
Various combinations of calcium aluminate cements with calcium sulphate. Portland cement and in some cases calcium hydroxide are conlmonly used to make cements that set and harden rapidly. Some such mixtures also show expansive behaviour (Section 10.2.1). A simple, rapidly sctting mix might contain 15-20%) ordinary Portland cement, 10-30% Cimcnt Fondu, 10-25% CH and 40-60% sand. More complex fornlulations, containing various admixtures, are used to produce self-levelling mortars. The setting times of all such mixtures depend greatly on the relative proportions. the characteristics of the individual cements and the mixing time and teniperature; only laboratory tests can predict the precise behaviour of a specific mix. Fig. 10.8 gives a general indication of the behaviour of neat pastes made from calcium aluminate and Portland cements. The rapid setting of the mixes high in Portland cement is possibly caused by preferential reaction of the gypsum with the calcium aluminate cement, and that of those high in calcium aluminate cement by the increase in pH due to the presence of the Portland cement. In addition to C-S-H and CH, the hydration products can include C4AH,, and stritlingite (C,ASH,). Mixes of CA with suficiently high proportions of calcium sulphate yield ettringite (C49).
10.1.I 1
Reactions of calcium aluminate concrete with external agents
Concrete made with calcium aluminate cement at a properly low w/c ratio is highly resistant to sulphate solutions, sea water or dilute acid solutions with pH > 4, including natural waters in which CO, is the only significant solute. Resistance may extend to pH 3 if the salt formed is of sufficiently low solubility. Midgley (M96) showed that, for fully converted material exposed to a sulphate ground water for 18 years, penetration with formation of a substituted ettringite was limited to a depth of 5 mm. These properties are consistent with Lea's (L6) view that the resistance is due to the formation of a protective coating of alumina gel, coupled with the absence of CH. No fundamental studies, e.g. on microstructural effects, appear to ha\e been reported. In contrast, resistance to attack by alkali hydroxide solutions is low, probably because the hydrous alumina is dissolved. Attack is accentuated if CO, is also present, as can occur if the calcium aluminate concrete is in contact with Portland cement concrete and exposed to atnlospheric CO, (R48). For CAH,,, the following reactions occur:
-
2Al(OH),-
+ CO,
-+
2AI(OH),
+ CO,'- + H 2 0
(10.4)
334
Cement Chemistry
The O H - ion t h ~ l sacts as a carrier for atmospheric C 0 2 and is efEctivelq regenerated; the CAH is decomposed giving CaCO, and hydrous alumina. This effect is known as alkaline hydrolysis. A similar proccss occurs with C,AH, or calcium aluminate concretcs in which conversion has occurrcd (B 132). In contrast to the above. carbonation in the absence of alkali increases the strength of calcium aluminate cement concrete (R49,P47). In sutticiently porous material in which conversion has occurred, some of the C,AH, reacts to give calcite and hydrous alumina (P45), but in dense concrete the extent of penetration by CO, is small, probably because the pores d o not contain the water necessary to promote the reaction (R49) and because they become blocked by CaCO, and alumina gel (B132). These observations support the general conclusion that the high resistance of dense calcium aluminate concretes to various forms of attack is at least in part due to effects that impede or prevent the ingress of the attacking substance. In addition to the blocking of pores by reaction products, these effects could include the rapid removal of incoming water by continued hydration.
,,
10.1.I 2
Refractory castables
Refractory castables can be categorized as cement castables ( I 5-25% calcium aluminate cement), low cement castables (5-10% cement) and ultra low cement castables having proprietory formulations that include under 5% of cement, microsilica and other admixtures. Depending on the service temperature and application, aggregates include exfoliated vermicul~te.chamotte, sintered bauxite, brown or white fused alumina, chromite, etc. A very simple formula for a wall more than 50 mm thick and for use at a hot face temperature up to 1650'C might contain 14% of '70% AI,O, cement', with 86% of a suitably graded aggregate of brown fused alumina and 0.2% of trisodium citrate on the weight of cement; the w/c ratio must be determined by an empirical test, as for a rapidly hardening mix, but would be G0.4. When the concrete is heated, the hydration products undergo conversion and dehydration. The strength decreases to a minimum value at a temperature usually between 900•‹C and 1 100•‹C, though if the cement content is sufficiently high it is still enough for ordinary applications. At higher temperatures, sintering reactions occur between the cement and aggregate, and the ceramic bonding that results causes the strength and resistance to abrasion to increase. The compounds formed depend on the compositions of the cement and aggregate, but typically include such high melting point phases as CA,, CA, and spinel (MA).
Calcium Aluminate, Expansive and Other Cements
335
10.2 Expansive cements
10.2.1
General
A normal Portland ccmcnt concrete expands during moist curing. but this effect is outweighed by the drying shrinkage that occurs on subsequent exposure to an atmosphere of normal humidity. If the concrete is restrained. the resulting tensile stress may be sullicient to crack it. In a n expansive cement. the expansion is increased. so as to balancc or outweigh the subsequent contraction. Expansive cements, which were lirst described by Lossier (L48), may be categorized as shrinkage-compe~is~~ted or sclf-stressing cements. In a shrinkage-compensated cement, the ob.ject is to balance the drying shrinkage to prevent cracking. In practice, the expansion is restrained by reinforcement and a small compressive stress of 0.2-0.7 MPn is created in the concrete, which should preferably remain under slight compression after the nornial drying shrinkage has occurred. Practical uses of shrinkagccompensated cements have included such structures as multistorcy cur parks or water storage tanks, in which i t is desirable or essential to climinatc water leaks. In a self-stressing cement, a larger expansion is produced, with the objcct of achieving greater stresses in the reinforcement and the concrete to permit the production of thin, strong articles or structures, as in nicchanicnl prestressing. Estimates of the niaximum stress that can be safely achieved have ranged from 3.5 MPa (M97) to 8 MPa (W30). Because the concrete expands triaxially, careful design and choice of curing conditions. including if necessary triaxial restraint, are essential. Self-stressing cements havc been used both in the nianuk~ctureof precast pipes and other units and Ihr in situ applications. In any expansive cement, the amount of expansion and the period during which the expansive reaction occurs are critical. The reaction must take place after the concrete has developed some strength, but before exposure to a reduced R H places it in tension. which could cause irreversible damage.
10.2.2 Types of expansive cement Probably a majority of practical expansive cements have depended on the modification of a Portland cement in such a way as to increase the formation of ettringite. Single expansive cement clinkers can be made, but i t has been more usual to produce admixtures that are blended or interground with a normal Portland cement or clinker. Blending has the advantage that the
336
Cement Chemistry
fineness of the expansive admixture can be optimized, usually by grinding i t more coarsely so as to delay the expansive reaction until the cement has developed the necessary strength. I t also allows a single admixture to be used in differing proportions to make both shrinkage-compensated and sclfstressing cements. In a widely used US terminology, expansive cements based on Portland cenient and calciunl sulphate are categorized as Types K, M or S. according to the source of the additional A120, that is required. Type K cements. developed by Klein and co-workers (K49). contain C,A,S as an essential ingredient. They are usually produced by intergrinding or blending an ordinary Portland cement clinker with additional gypsum or anhydrite and on expansive clinker containing C,A,S. The composition of the expansive clinker can be varied considerably to utilize the raw materials most economically available; the clinkers made in the USA typically contain 8-50% of C,A,S with some anhydrite and usually some free lime (M97). Thc proportions of expansive clinker and of gypsum or anhydrite or both that are added to the Portland cenient depend on the composition of the expansi\e clinker and the degree of expansion required. Mehta and Polivka (M97) found that Type K shrinkage-compensated cements made with Portland cement clinkers of low or moderate potential C,A content and containing 6 % or more of SO, resisted chemical attack by sulphates, but that Types M and S cement did not. Table 10.3 includes some analyses of Type K cements. Table 10.3 Conlpositions of Type K expansive cements produced in the USA
(M97) Application
MgO AI,O, SiOz
SO,
CaO
Fe,O, Loss
Shrinkage compensation Shrinkage compensation Self-stressing
0.7 3.4 2.7
6.2 6.5 12.4
62.4 61.4 57.6
2.8 I . 1.6
4.8 5.7 8.4
21.6 18.8 14.6
1.1 1.5
1.9
Type M cements contain Portland cement, a calcium aluminate cement and additional calcium sulphate. They have been extensively studied in the USSR. Mikhailov (M98) described a self-stressing cement conlposed of Portland cement, calcium aluminate cement and gypsum typically in the proportions 66 : 20 : 14 and used to produce precast units. An initial cure in air followed by hot water allowed strength to be developed without formation of ettringite, which occurred during a subsequent cure in cold water. Mixtures using alumina slag as the source of the additional AI,O, have also been used, and require only curing in air followed by cold water (M97,M99). Mixtures of calcium aluminate cement with gypsum (X2) or with gypsum
Calcium Aluminate, Expansive and Other Cements
337
and C,AH,,, the latter made from calcium aluminate cement and lime (M98), have also been employed. Type S cements are Portland cements high i n (',A and with suitable contents of calcium sulpliate; they have found little litvour as they are too tliflicult to control. The AI,O, has also been supplied in forms other than tliosc mentioned above. Impure alunite [KAI,(SO,),(OH),], which occurs as ;I natural rock, has been used, either after calcination ( V 6 ) or uncalcined (W30). In the latter case it was mixed with Portland cement clinker. anhydrite and pfa or slag and was found to dissolve relatively slowly. thus suitably delaying the expansive reaction. The possibilities of using free lime or pericluse as expansive agents have also been investigated. Kawano ot t i / . (K50) and Long (L49) described clinkers in which substantial proportions of free lime are present as inclusions in alite. In both cases, the raw mix included anhydritc. Long in\estigated and described the processes by which this nlicrostructure was formed.
10.2.3
Mechanism of expansion in Type K cements
In a Type K cement, the formation of ettringite from C,A,S ma) be represented by the equation
2610 kgm-" CSH,, Consideration of the relevant densities (c,A,s, 2310 kg n1r3; CH, 2240 kg m-" CC,As,~,,, 1780 kg n i - 9 shows that the solid volume is approximately doubled. Water could be drawn in from the outside, and its volume is irrelevant. As the case of the normal hydration of Portland cement shows, an increase in solid volume is not in itself a sutficient cause ofexpansion. I f sufficient pore space is available in which the additional product can be deposited and if the deposition occurs in that pore space without disturbing the surrounding matrix, no expansion will occur. Conversely, a small increase in solid volume may produce a large expansion i f by forcing parts of the surrounding lnicrostructure apart it causes the porosity to increase. There is thus no simple relation between the change in solid volume and the expansion of a porous, or potentially porous. material. Any attempt to understand the expansive process niust thus consider the manner in which the deposition of the products affects the microstructure. The production of ettringite can cause expansion, but does not necessarily do so; in a supersulphated cement (Section 9.2.9), much ettringite is formed, but there is no marked expansion. Comparable amounts of ettrink' 71te are reported to be formed when ordinary or sulphate-resisting Portland cements
338
Cement Chemistry
are placed in sodium sulphate solutions (H59). but the expansion is much greater with the ordinary Portland cement. For expansive cements, Nakamurn c / N / . (N19), M e h h (M 100) and Bentur and Ish-Shalom (B133.19, lI0.13134) all recognized that the niode of formation and morphology of' the ettringite must be important. In suspensions of C,A,S, cs and CH at high CaO concentrations. the ettringite is formed as a tine-grained coating on the C,A,S particles. but in the absence of C H it forms long. thin needles (N I9.023.M 100). In contrast, a study in which two ditrerent Portland clinkers were mixcd uith gyp.sum and an expansive clinker low in free lime showed that increase i n thc conrent of free lime in the cement increased the rate of ettringite formation but had little effect on ettringite morphology (KSI). In pastes containing C',A,S. expansion occurs if CH is present. but not if it is absent. w e n though ettringite is still formed (N 19.073). Expansion also requires an adccluatc wpply of water; sealed curing either limits (N I Y ) or prevents ( M 100) it. etcn though the w/c ratio is sufficiently high to allow a substantial quuntit! of ettringite to be formed. Bentur and Ish-Shaloni (B133,IY.llO.B134) stitdied mixtures of C',.A,S. cs and CH in pastes, alone or mixed with C,S. They found that expansion does not begin until a certain critical degree of reaction, which was about 50% at 20 C, had been reached. Using SEM, light microscopy and mercury intrusion porosimetry, they confirmed that the ettringitc was formed as porous coatings around the grains of C,A,S, which were tli~tscfli.cti\.cly caused to expand; expansion began when the expanding particles cnmc into contact and exerted pressure on each other. In the absence of restraint. thcsc particles could be treated as sphcres, but under restraint they cspanciccl in the directions in which the restraining forces were least. Essentially the same mechanisni was described by Ogawa and Roy (016,024). who fo~tndthat the ettringite was formed initially as very small, unoriented crystitls. After the degree of reaction (defined by the quantity of ettringite formed) had reached about 50n/0. the ettringite developed as radial growths of longer crystals. Contact between the ettringite layers surrounding the C',A,S particles, and the associated expansion, began only after this stage had been reached. The change in ettringite morphology was considcrcd to be an important first step in causing expansion. Cohen (C55) reviewed theories of the cxpansion mechanism. Probably a majority of workers (e.g. Refs I9,I 10,016,024,B133) have attributed espansion to forces exerted by the growth of the ettringite crystals. Of other theories, the most significant is that proposed by Mehta (M 100). who attributed it to imbibition of water by the gelatinous layer of colloidal ettringite. On this hypothesis, expansion does not occur with the larger crystals formed at low CaO concentrations because these d o not form
Calcium Aluminate. E x ~ a n s i v eand Other Cements
339
aggregates that imbibe water. While this mechanism may contribute to the expansion, Ogawa and Roy's observations suggest that i t is not the sole or even the niajor cause.
10.3 Other cements
10.3.1 Very rapidly hardening cements
Various methods have been used to obtain cements that set and harden rapidly. They include the use of Portland cement with admiuturcs and ol' mixtures containing both Portland and calcium aluminate ccmcnts. described in Sections 1 1.5 and 10.1.10, respecrivcly. Another approach ha\ been the manufacture of clinkers containing either C , ,A,.CaF2 or C',A,S. both of which hydrate rapidly under appropriate conditions wit11 the formation of ettringite. Regulated-set cement and jet cement are modified Portland cements i n which the normal aluminate phase is replaced by C , , A 7 . C a F 2through thc use of a raw mix containing CaF2. Ucliikawa and Tsukiyama ( U 2 I ) g a w chemical (Table 10.4) and phase compositions of two jet cements. Hot11 contained approximately 60'% of alite, 20% of C , ,A;CaF,. I?;, ol' belitc and 5% of ferrite. Admixtures are required to control the rate of reaction of the C , , A 7 C a F , and the nature of the products. One o f t h e cements included a proprietary retarder based on citric acid. together with 2% of CaC'O,. The other contained 2.5% of hemihydrate. In cach case. Na,SO, ( 1 ' 1 1 ) ) and anhydrite were also present. The specific s u r f x e areas were around 550 mQg-'. Chemical and phasc compositions of rapidly hardcn~ngmatel-ials cvntaining C , ,A;CaF, or c,A,S
Table 10.4
M:~terial
NazO MgO A120, SiOz SO., KzO CaO E'e,O, F
Set cement
0.6
tligh strcngth (clinker)
-
0.8
11.0 28.1
13.9 1 1 . 1 0.4 58.9 1.7 11.2 7.1 52.0 1.6
Loss Ref.
1.0 0.2
U21 S 10 1
Uchikawa and co-workers (U21 -U23) studied the hydration reactions of C , , A , C a F , and jet cements, including the effects of various ncimixtures. A wide variety of methods was used. For the two cements mentioned 4, bo\'e. hydrated in pastes at 20"C, the main product at 3 h was ettringite. At this stage, 40-45% of the C , , A , C a F , had reacted. SEM showed needles of ettringite, 0.5-1.0pm thick, growing from the particles of clinker and
340
Cement Chemistry
anhydrite to form a closely knit, interlocking structure. Comparable mortars of the same age had compressive strengths of around 10 MPa. The quantity of ettringite formed was such that a high proportion of the anlij,dritc must have reacted. Subsequently, the content of ettringite tended to dccrcasc slightly and some nionosulphatc was formed; the ettringite rccrystollizcd. giving larger crystals. The alitc hydrated to give C S H: calculations bnscd on assunied reaction stoichiometry indicated that the reaction \vas much faster than in a Portland cement, with about 65%0 reacting in I day. The compressive and bending strengths increased rather slowly between 3 ti and 24 h, but thereafter more quickly, and by 7 days the fornier were 37 44 MPn. The type of retarder (citric acid or hemihydrate) considerably alkcted the hydration reactions and the pattern of strength development. The Iluoricic ion was believed to enter an aluminium hydroxide fluoride. Cements that set and harden rapidly can also be made in which the principal constituents are C4A,S and belite. Table 10.4 includes the chcmicnl composition of a clinker described by Sudoh cJ/ (11. (S 10 I ). The clinker. made from limestone, byproduct gypsum, aluminous materials and clay. was burned at 1300•‹C.The presence of a limited amount of free lime was found to be necessary to achieve rapid strength development, and conditions such that all the SO, was bound as C,A,S were required to avoid atmospheric pollution. The phase composition was given as: C,A,S. 54%; C,S. 37%: C,AF, 5%: free lime, 9%. The clinker was interground with byproduct anhydrite to a specific surface area of 400 m 2 kg- '; the optimum niolnr ratio of total SO, to A1,0, was found to be 1.3-1.9 (20-~25'YoSO,). X R D and SEM of I-day-old pastes showed the main product to be cttringite. the needles of which were 1-2 pm long and formed an interlocking network. A mortar made with the cement containing 20% SO, developed compressive strengths of 27 MPa at 3 h, 49 MPa at I day and 59 MPa at 28 days. The strength development at later ages was attributed to hydration of the C,S. For supporting and sealing the walls of roadways in coal mines. a material is required that can be transported as a slurry which remains stable for long periods while stationary in a pipeline but which when placed develops a modest compressive strength within a few hours. This can be achieved by enlploying two slurries that react and harden on being mixed. I n one such system, one slurry contains a clinker containing C,A,S and the other contains anhydrite, CH, bentonite and alkali metal salts (L50). The C,A,S clinker, which can also be used as an expansive admixture with Portland cement, hydrates only slowly in the absence of CH. In the second slurry, the bentonite keeps the other solids in suspension and the alkali metal salts act as accelerators. In most of the cements described in this section, ettringite is an important hydration product, but, as in supersulphated cements, its fortilation does not
Calcium Aluminate, Expansive and Other Cements
341
cause significant expansion. It is presumably formed either at a stage when the matrix of other phases is readily deformable, or in crystals which grow in a manner that allows them to be accommodated in an existing pore structure without causing disruption.
10.3.2
Low-energy cements
Environmental pollution, including the emission of CO,, and the depletion of resources of fossil fuels and raw materials, are among the most serious problems confronting humanity today. As Boldyrev (B135) and others have stressed, the cement industry is a major user of both energy and raw materials, and can make very considerable contributions to the amelioration of these problems, especially through increased energy efficiency and by the use of industrial wastes in place of natural raw materials. Major improvements in the energy efficiency of Portland cement production have already been made, but more should be possible and the problems of raw materials and of the utilization of wastes produced by other industries remain. Locher (L51) reviewed ways of decreasing the energy consumption in cement manufacture to values substantially below those characteristic of modern Portland cement production. They included the use of fluxes to lower the burning temperature, the production of cements containing more reactive forms of belite or based on belite together with highly reactive phases, replacement of alite by the chloride-containing phase, alinite. and the use of composite cements. The burning temperature for production of Portland cement clinker can be decreased by about 150•‹Cthrough the use of fluxes, but opinions have differed as to the energy saving thereby obtainable. Klemm and Skalny (K52), who reviewed the subject, estimated it at 630 kJ kg-'. Christensen and Johansen (C56) considered that this figure, while possibly realistic for an inefficient, wet process kiln, was unlikely to be so for a modern, precalcinerpreheater kiln, in which heat recovery is efficient. They considered a value of 105 kJ kg- ' more realistic. The major component of the enthalpy change in clinker formation arises from the decomposition of calcite, and a large saving could be achieved if the proportion of belite in the clinker could be increased. This is only of practical value if the belite can be rendered more reactive or if its low reactivity is compensated by including also some highly reactive phase, such as C,A,S. Reactive forms of belite have been obtained in the laboratory by low-temperature methods (R50,V7) but it is not clear how the latter could be effectively utilized in practice. Rapid cooling of a clinker (e.g. at 1000 deg C min-') increases the reactivity of belite, at least in part, by preserving an a'
342
Cemenr Chemistrv
polymorph (G77.SI02) and may olTcr a more practical approach. I t is apparently difficult to obtain adequate early strength. but clinkers containing around 48'X) of u'-CIS and giving compressive strengths of up to about 20 MPu at 3 days have been made on a pilot plant scale (S102). The clinkers were quenched in water. and in calculating the heat balance it would be necessary to take into account the extent to which the heat contained in them is recoverable. Thc properties of one of the rapid-hardening cements described in the prwioi~ssection show that good strength development can be obtained from cements in which the major constituents are belite and C,A,S. Cements of a similar nature but with much higher contents of belite have been described. A n alumina bclite cement (212) was obtained by burning at 1300 'C ofa mix intermediate in composition between those of Portland and calcium aluniinnte cements. with some gypsum. I t contained 62--64% of belite, with CA. C , ,A,, C,A,S. and C,S,S. and was claimed to give good strength development and resistance to sulphate attack. Porsal cement (V8) is essentially similar and was made using CaF, as a mineralizer. Cements high in F e 2 0 , have been described. containing. for example, 30% C2S, 30% ferrite. 20% iit C,A,S and 20% CS, and were obtained by burning suitable niixt~~res 1200 C (MIOI). The ferrite formed at this temperature is likely to bc reactive. and the clinker is probably easy to grind. Alinite cements are discussed in the next section. Some other approxhes and aspects may be considered. The partial substitution for limestone of raw materials that do not require decarbonation was noted in Section 3.2.1. The compositions of most of the available materials, such as slags, that niight be thus used limit the extent to which this is possiblc in the production of a normal Portland cement clinker. but a wider range or higher content of waste materials niight be usable in making other types of clinker. Mehta (M 101) noted the possibility of using byproduct gypsu~ii,red mud (from aluminium extraction), slag and pfa in producing high-iron cements such as those mentioned above. Some limestones unsuitable for making a normal Portland cement clinker could be used to make clinkers higher in belite. Burning at a reduced temperature might be expected to decrease the I'orniation of NO,. In appraising the energy savings obtainable with any new type of cement. it is essential to consider not only fuel consumption, but also the total energy requirements. including. for example, those of winning and transporting raw materials and of grinding raw materials and clinker. The comparison sho~lld also be with modern, dry process plants. Locher (L51) emphasized the further need to consider the properties of the product, including not only the 7- or 28-day strength of the concrete, but also the durability that will be obtained given the curing conditions likely to exist in practice. If the early
Calcium Aluminate, Expansive and Other Cements
343
strength is low. the surface regions are liable to dry out before the cement in them has hydrated sufficiently, resulting in high porosity and poor durability. He also concluded that, with the low-energy cements so far investigated. the energy savings are probably no greater than can be achieved by use of composite cements containing slag, pfa, natural pozzolanas or fillers, the behaviour of which is now well understood.
10.3.3 Alinite cements A cement in which the alite of Portland cement is replaced by alinite. a structurally related phase containing essential M g 2 + . Al"+ and CI-, was developed in the USSR by Nudelnian and others and patented in 1977. An otherwise essentially normal raw mix is modified by adding CaCI,. which allows the maximunl temperature in the kiln to be reduced to 1000- 1100'C. Considerable energy savings are claimed, due to lower heat losses. a lower enthalpy of reaction and easier grinding of the clinker. Bikbaou (B136) gave the phase composition as alinite, 60-80%, belite, 10-30n/n, C , A,.CnCI,. 5lo%, ferrite. 2-10%. and discussed the process of clinker formation. Ca2CI2(CO,) and spurrite were reported to form as intermediates and the alinite crystallizes from a CaCI, melt. The process of formation has been further discussed (B137.N20,A24). Locher (L51) gave a survey of more general information, based partly on the patent data. The preparation of the pure compound has been described (M 1 02,Vg). The crystal structure of alinite has been determined ( I I I,V9). The constitutional formula was found to be Ca,,,Mg,,,~,,,(Si04),,4(A104) ,.(, 0 CI. where denotes a vacancy; limited variations in the Mg/Ca and Al Si ratios are possible (V9). Alinite is tetragonal, space group 142111, with tr = 1.0451 nm, c = 0.8582 nni, Z = 2, D, 3010 kg m-"V9). and is isostructurnl with the silicate sulphide mineral, jasmundite, C a , ,(SiO,),O,S (D43). A Br-analogue has been prepared (K53).
,
,,,
-
Table 10.5 Conipositions of alinitc clinkers (A24) --
MgO AI,03 Si02
2.0-2.2 4.4-4.6 22.7-23.3
CaO
62.5-63.9
Fc,03 3.6-4.8 CI (total) 1.0-2.9
CI (frcc) Ignition loss Frcc lime
0.04-0.12 0.7-1.0 <0.5
Agarwal cJr ul. (A24) examined two alinite clinkers and further studied the process of their formation. The compositional range (Table 10.5) shows that. compared with typical Portland clinkers, they are of low LSF and low AR. The microstructures resembled those of typical Portland clinkers. X R D
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Cement Chemistry
confirmed the presence of C , ,A,CaCI, and showed that a calcium chloride orthosilicate was also present. On heating the clinker to 1400 C. (he alinite decomposed, but much of the chlorine was retained. Von Larnpc PI (11. (V9) also showed that alinite decomposes slowly at 800•‹Cand rapidly at 1430 C. The calcium chloride silicate most likely to be formed. in addition to alinite, is Ca,SiO,CI,. A study of its hydration behaviour ( K 5 4 ) showed it to be relatively highly reactive, C-S---H and CH being detectable by X R D in little over 1 h. Ca,CI,(OH),~H,O was also formed at w/s = I . though not at w/s = 10, and might thus be expected to be a product in a paste. The hydration reactions of alinite cements do not appear to have been reported, but it may be surnlised that they are similar to those of Portland cements, with the probable addition to the products of C,ACaCI2.I0H,O and possibly also of Ca,CI,(OH),.H,O. The CI- is, however. not wholly and permanently combined in these or other hydration products. I n tests on reinforced concrete, serious corrosion had occurred within 5 years. showing the cement to be unsuitable for such use (L51).
Admixtures and special uses of cements
11 .I
Introduction
Admixtures for concrete are defined as materials other than hydraulic cements, water or aggregates that are added immediately before or during niixing. Materials such as grinding aids that are added to cement during manufacture are called additives. The most important admixtures are ones added to accelerate o r retard setting o r hardening, to decrease the quantity of water needed to obtain a given degree of workability, or to entrain air in order to increase the resistance of the concrete to damage from freezing. Mineral additions (Chapter 10) are not included here in either category. Many materials have been used as admixtures and the potential of many more has been examined. The literature, including the patent literature and other descriptions of proprietary materials, is voluminous; a useful review published as long ago as 1980 includes over 600 references (M 103). We shall only attempt to describe the effects of the most important classes of admixtures in general terms and to discuss the chemistry underlying those effects at a fundamental level. Grinding aids are similarly considered. The rest of the chapter deals with the hydration chemistry of Portland and composite cements at temperatures outside the range 15-25'C, including that of autoclave processes, and with specialized uses of cements in casing oil wells and in making very high strength materials.
11.2 Organic retarders and accelerators 11.2.1 Retarders
Admixtures that retard setting are of value for concreting in hot weather and other purposes, including oil well cementing (Section 1 1.8). Many organic miterials have this property; sucrose, calcium citrate and calcium lignosulphonate are examples. They are effective in low concentrations, suggesting that they act by adsorption. Taplin (T48) noted that most contain one or more groups in which oxygen atoms are attached to adjacent carbon atoms,
346
Cement Chemistry
such as --CO.C(OH)-, or are otherwise able to approach each other closely. He also noted that adsorption on either a clinker phase or a hydration product niight cause retardation. in the latter case through interfering with growth. Young (Y8), reviewing the evidence, concludcd that adsorption on products, especially CH, was the prime cause of retardation. He considered that adsorption on anhydrous phases also occurred and that. with pure C,A or C,S, i t caused an initial acceleration by favouring dissolution. Ramachandran 0 1 (11. (R32) discussed factors af'ecting the cflicacy of a retarder with cement. They included the ratio of retarder to cement. the time at which the retarder is added, the temperature and the compositions of the cement and of the mix. The time of initial set increases with the content of retarder and generally decreases with temperature and cement content. In a typic:tl case, addition of 0.1 % of sucrose on the weight of cement niight increase the time of initial set from 4 h to 1411, while a 0.25% addition might delay i t to 6 days. Sufficiently high additions can delay settins indefinitely. Retarders are more effective with cements low in aluminate. because the latter, or their hydration products, consume disproportionate amounts of retarder. Some are more effective with cements low in alkali. perhaps because the latter destroys them. Retarders are most efTective if added 2--4 min after mixing, because the aluminate has by that time reacted to some extent with the gypsum and consumes less retarder. The interactions between these and other factors are sufficiently complex that the efrect of a specific retarder on a given mix should be pretested before i t is used on a job. This applies to admixtures in general. 11.2.2 Mechanism of retardation
Tho~nasand Birchall (T49) obtained strong support for the hypothesis that retardation arises from adsorption on a hydration product. They determined analytical concentrations of calcium, silicon, alumini~m.iron and O H in the solution phases of slurries of cement (w/c = 2) with and without additions of sucrose or other sugars, at ages up to 7 h (Fig. I I. IA and B). In the absence of a retarder, the concentrations of silicon, aluminium and iron were all very low (Si = 0.04 f 0.01 mmol 1 Al 6 0.01 mrnol I -'. Fe 6 0.01 ninlol 1 1 ) , and Ca/Si ratios were in the order of 1000. In the presence of sucrose at 50 mmol I - I , the calcium and O H - concentrations were somewhat increased, but those of silicon, aluminiuni and iron were increased by factors of up to 500, so that the Ca/Si ratio at 2 h had fallen to 6. The results were explained (T49,B138,T50) by supposing that poisoning of the surfaces of the hydration products allowed the Ca2+ ions to coexist in solution with the silicate, hydroxoaluminate and hydroxoferrite ions at concentrations that were impossible if the solution was in equilibrium with
-',
Admixtures and Special Uses of Cements
347
-200j -
S=SUCROSE (50 mmol I-')
7
0
, ]
I I (CI 0.33 1 10 100 TlME ( M I N U T E S )
E=EDTA (50mmol I-') C= CONTROL
13
I
TlME ( MINUTES 1
-
Si
-
0.33
1 10 100 TlME ( M I N U T E S I
Fig. 11.1 Concentrations in the aqueous phase of Portland cement hydrated at w/c = 2.0 in water or in sucrose or EDTA solutions. After Thomas and Birchall (T49).
348
Cement Chemistry
the unmodified hydration products. N M R studies showed that Si-sucrose complexes were not formed. The results of a similar study in which EDTA was used in place of sucrose further supported this conclusion (T51). The silicon concentration in this case increased rapidly during the first 20 s. but soon decreased to a u l u e similar to that in the control (Fig. I I . IC). EDTA is a strong complexing agent for C a 2 + in alkaline solutions; it therefore effectively removes the Ca" from the system, and allows the silicate ion concentration to increase. Because it acts in the solution and not on a surface, a stoichiometric quantity ( I EDTA anion per C a 2 + ion) is required, and the effect ceases when this is used up. Sucrose, in contrast, acts by adsorption, and much less is required. In accordance with these conclusions. EDTA, though an excellent complexing agent for C a 2 + , has a negligible effect as a retarder when used in concentrations typical for such admixtures (T48). Thomas and Birchall (T49) summarized and extended data on the comparative retarding effects of different sugars. They distinguished three categories. Trehalose and u-methyl glucoside are inefrective as retarders: both are non-reducing sugars containing only 6-membered rings. Glucose. maltose, lactose and cellobiose, which are reducing sugars containing only 6membered rings, are good retarders. Sucrose and raffinose. which are outstanding retarders, are non-reducing sugars containing 5- and 6-membered rings. The ability to solubilize CH was shown to be highest for maltose, lactose and cellobiose and lowest for trehalose and u-methyl glucoside, and thus does not run parallel to retarding ability. This is shown also by the behaviour of EDTA, noted above. Experiments in which calcium oxide or hydroxide was treated with sucrose solutions (B138) showed that a I : 1 Ca-sucrose complex was formed. I t was concluded (T49,B138) that the retarding species was R 0 Ca--OH; where R represents the sucrose anion, which was incorporated into the surface of a growing particle of CH or C-S-H, thereby inhibiting growth. The calcium was considered to be attached to the fructose (5membered ring) part of the molecule. Trehalose and u-methyl glucoside were considered to be ineffective because their ability to complex calcium is very wjeak. The reducing sugars were considered to be less effective than sucrose or trehalose because opening of the ring allowed them to chelate calcium. giving complexes that were less readily adsorbed. Birchall and Thomas (B138) explained the inhibition of growth of C-S--H by assuming that the CaO, layers are the first parts of this material to be formed, and could thus be poisoned by the retarder. Taylor (T52) earlier proposed a similar mechanism for the growth of C-S--H: starting from mononuclear complexes, such as [Ca(H,O),(OH)]+, repeated condensation through sharing of edges and elimination of H,O molecules leads to the
Admixtures and Special Uses of Cements
349
formation of a CaO, layer, which serves as a matrix for the formation of Si,O, groups, and ultimately of dreierketten. He considered it unnecessary for a complete CaO, layer to be formed before silicate ions can begin to condense with it. I t is most unlikely that the Si-0 parts of the structure could form before the Ca-0 parts because, although a colunln or layer of linked Ca-0 polyhedra can exist without dreierketten, the reverse is not true. The manner in which the silicate tetrahedra condense is governed by the coordination requirements of the metal cations, and is entirely different if the Ca2' is replaced, for example, by Mg2+. Jennings c.t ul. (533) found that with C,S, the retarding efTect of sucrose was greatest if it was dissolved in the mixing water. This contrasts with the situation for cement. They concluded that the retarder was incorporated into the initial product and impeded its transformation into a second product. This explanation is compatible with that discussed above if incorporation is taken to mean adsorption on nuclei or growing crystals of the second product. Although there may well be a phase transformation, this evidence does not seem to demand one. Even without one, the retarder would be more effective the sooner it was added, because a smaller area would need to be poisoned. The situation with cement differs due to the competing effect of the aluminate phase. The broad conclusion indicated by the evidence considered in this section is that organic retarders are substances that are rezdily adsorbed on to the surfaces of growing particles of hydration products, especially C S - H , and which do so in preference to complexing C a Z +ions in aqueous solution. The monodentate mode of attachment proposed by Thomas and Birchall (T49) explains the differences in behaviour among sugars, but its applicability in this respect to a wider range of substances has yet to be examined. I t does not appear to account for the observation that the etrective retarders are compounds in which two oxygen atoms are located on adjacent carbon atoms, or are otherwise able to approach each other closely.
11.2.3
Practical retarders
The retarders most widely used in practice appear to be hydroxy carboxylic acids or their salts, such as citrates or heptonates, and lignosulphonates. Because of the low concentrations required, they are commonly added as solutions. Calcium or sodium lignosulphonates are waste materials from the manufacture of wood pulp, when they are formed by the breakdown of lignins. Their molecules are random, three-dimensional polymers. with molecular weights of up to 50 000, probably spherical and with many -SO, groups on their surfaces. The molecular structures are not completel!
350
Cement Chemistry
known, and the element shown in Fig. 1 1.2 is purely illustrative of the types of local grouping that have been identified. Unpurified comnlercial lignosulphonates contain substantial proportions of sugars and related salts of sugar acids, such as gluconates, which are at least partly responsible for their retarding properties. Differing views have been expressed concerning the extent of the contribution from the lignosulphonate niolecules themselves. Milestone (M 104) and others have concluded that the retarding efkct with C,A is largely due to the sugar acids, but Ramachandran (R5l) found that, in low concentrations with cement, purified lignosulphonates were almost as effective as the unpurified material.
Fig. 11.2
11.2.4
Typical clcnicnt o f structure from a lignosulphonatc anion.
Organic accelerators
Calcium formate accelerates setting and hardening in a manner similar to that of many inorganic salts (Section 11.5). Oxalic acid. added to C,S, increases the extent of the initial reaction, but has no significant eft'ect on the length of the induction period or the subsequent major stage of reaction (013). Calcium oxalate is precipitated, and the concentration or chemical potential of C a 2 + ions close to the surface of the C,S is thereby kept low until all the oxalic acid has been used up. Triethanolaniine is added to some admixture formulations to decrease the retarding effect of the water reducer. The chemistry of its action is not well understood. Ramachandran (R52) found that, added as the sole admixture in amounts greater than 0.1 % I , i t accelerates the reaction of the aluminate phase but markedly retards that of the silicates. At 0.594, initial set occurs within a few minutes, but final set is delayed to over 24 h. This behaviour appears to be qualitatively similar to that of glucose, which also accelerates the initial reaction of the aluniinate
Admixtures and Special Uses of Cements
351
phase, though it delays the subsequent replacement of ettringite by monosulphate and greatly retards the hydration of the silicates.
11.3 Air-entraining agents and grinding aids
11.3.1 Air-entraining agents The ability of concrete to resist damage from freezing of the pore solution is increased by introducing voids into which the latter can expand. Air is entrapped on normal mixing, but is of limited value because the resulting voids are too large and too widely spaced. By adding a suitable surfactant, additional air is entrained and produces a stable foam, which persists in the hardened paste in the form of closely spaced, spherical pores. Air entrainment also improves workability. T o avoid undue loss of strength. the total volume of pores should not be larger than necessary; the pores should therefore be small. The pores should not readily fill with water if the paste is saturated; small, isolated pores are most likely to meet this requirement. Klieger (K55) discussed the separations and total volunles of pores needed to provide adequate frost resistance. Typically, the pores are 10-250 pm in diameter and the average maximum distance from any point in the paste to thc nearest void, called the void spacing factor, is about 150 p n . Air-entraining agents are characteristically long chain molecules with a polar group at one end, and which therefore become concentrated at the airliquid interface with their polar groups in the liquid and their non-polar parts out of it. Bubbles are formed by entrapment of air, but in the absence of the air-entraining agent are mostly destroyed because of the surface tension of the liquid. The air-entraining agent lowers the surface tension and stabilizes them. The inner surface of the bubble is thus composed of hydrophobic material formed by the non-polar parts of the molecules which acts us a barrier to the entry of water during mixing or placing and on any subsequent occasion when the paste is saturated. Air-entraining agents may be simple compounds, such as sodium salts of fatty or alkyl aryl sulphonic acids, but some are more complex (K55). The quantity of the active ingredient used is typically of the order of 0.05% of the weight of cement. As noted earlier, the property of air entrainment overlaps in varying degrees with those of retardation and water reduction. Air entrainment increases the workability of an otherwise similar concrete; by allowing the w/c ratio to be decreased, this can partly or wholly offset the loss in strength arising from the presence of the air voids. Admixtures are known that decrease air entrainment; butyl phosphate is an example.
352
Cement Chemistry
11.3.2 Grinding aids Grinding aids are sometimes used in clinker grinding, to decrease the energy required to achieve a given fineness of grinding or throughput. Massazza and Testolin (M103) reviewed their use. They are surfactants of various kinds, of which aniines and polyhydric alcohols are possibly the most ctrective, and the amounts added are normally 0.01-0.1% on the weight of clinker. They appear to act mainly by decreasing agglomeration. though it has been suggested that they also aid fracture by preventing incipient microcracks from healing. There are conflicting reports on their effects on setting times, early strength and other properties. Grinding aids are clearly taken up by the anhydrous phases, and it is probably significant that the more effective are of different chemical types from retarders and water reducers, which are taken up mainly or entirely by the hydration products.
11.4 Water reducers and superplasticizers
11.4.1 Water reducers Water-reducing agents, also called plasticizers, allow a given degree of workability to be achieved at a lower w/c ratio. Conventional water reducers allow w/c to be decreased by 5-15%. To achieve greater decreases. they would have to be used at concentrations that would also cause excessive retardation. excessive air entrainment or flash setting. More powerful water reducers, called superplasticizers, allow w/c to be decreased by up to about 30%: these are considered in Section 11.4.2. Water reducers act by adsorption at the solid-liquid interface. in this respect resembling retarders. There is a wide overlap between the two properties, many retarders being in varying degrees water reducers and vice versa. Both properties also overlap to some extent with that of air entrainment. which is promoted by admixtures that act at the air-liquid interface. Conventional water red~~cers are typically added as solutions in concentrations of up to 0.2% on the weight of cement. Calcium and sodium lignosulphonates are widely used both as retarders and as water reducers. Their retarding effect can be decreased by treatment to lower the content of sugars and sugar acids, and can be further decreased or reversed by blending with an accelerator, or enhanced by blending with a stronger retarder. Compounds that decrease air entrainment may also be added. Other materials used as water reducers include salts of hydroxy carboxylic acids, hydrolysed carbohydrates, and hydrolysed proteins. In general. water reducers are effective with Portland. con~positeand
Admixtures and Special Uses of Cements
353
calcium aluminate cements. Many of the charactct-istics of retarders. described in Section 11.2.1, apply also to water reducers. and for the same reasons; thus, they are most effective if added a few minutes after mixing. and. in some cases at least. with cements low in aluminate phase and alkali. The workability of any fresh concrete decreases with time after mixing. but this effect. which is called slump loss. is more marked if a water reducer is used. The slump nevertheless remains higher than if the latter was absent. Slump loss is c a ~ ~ s eby d the slow commencement of the hydration reactions. and its increased magnitude in concrete containing water rcduccrs is probably due to the gradual absorption of the admixtul-c by the hydtxtion products. Delay in adding the ximixture until a few minutes after mixing minimizes it.
11.4.2 Superplasticizers Superplasticizers are also called high-range water reducers. The marked lowering of wjc ratio that they allow makes it possible to produce highstrength concretes. especially if thc mix also includes microsilica. Alternatively, the use of superplasticizers at normal w/c ratios allows the production of 'flowing' concrete that is self-levelling and can I-cndily be placed by such methods as pumping or continuous gravity feed thro~tgh;I vertical pipe. The greater erect compared with conventional water rcduccrs is duc to the fact that they can be ~ ~ s in e dhigher concentrations, which can bc over 1 ' X B on the weight of cement, without causing excessive retardation o r air entrainment, If used in similar concentrations to conventional water reducers. the degrees of water reduction obtained are also similar. Three principal types of superplasticizer are in common LM, salts of sulphonated melamine formaldehyde polymers (SMF), salts of sulphonnted naphthalene formaldehyde polymers (SNF), and modified lignosulphon;ite materials. Most of the available information relates to the first two t),pcs. which will alone be considered here. Both are linear anionic polymers \vith sulphonate groups at r e g ~ ~ l aintervals r (Fig. 11.3). The S M F materials arc high molecular weight polymers, their most effective components possibl!. having molecular weights of about 30 000. The S N F materials are less highl!, polymerized, and have chains some 10 units in length o r less. Both are conimonly added as solutions. Comn~ercialformulations often contain other substances added to alter setting bchaviour or for other reasons. Most of the characteristics of conventional water reducers are shown also by superplasticizers. at least qualitatively; thus, they are more efTective if added a few minutes after mixing, retard setting and increase slump loss. Retardation of set is slight with S M F but more marked with SNF. Slump .:13;
354
Cement Chemistry
loss is considerable with both types, and is accelerated by incrci~se in temperature. At 15"C. the enhanced fluidity typically persists for 30 60 min. Slump loss is lessened if the superplasticizer is added a few minutes after mixing, and can also be partly countered by including a retarder or. if practicable, by adding a second dose of s~~perplasticizcr before placing. Unlike conventional water reducers, superplasticizers used in normal concentrations d o not cause significant air entrainment. and may cvcn dccrensc the amount of entrapped air because of the greater fluidity of the mix. Airentraining agents can be added, and must be used in higher than normal concentrations.
Fig. 11.3 Repeating units of the structures of superplasticizcr-unions: ( A ) naphthalcnc forrnaldchyde condcnsatc; (B) mclaminc Ibrmaldchyde condcnsatc.
11.4.3 M o d e o f action of water reducers and superplasticizers
The fact that the water-reducing effects of superplasticizers arc similar to those c.f conventional water reducers used in similar concentrations suggests that the modes of action are similar, and that the essential difference is that the properties that limit the concentrations in which conventional water reducers can be used are weaker or absent in superplastizers. These propcrties comprise retardation, air entrainment and in some cases flash setting. It is widely agreed that water reduction is effected through improved dispersion of the cement grains in the mixing water; flocculation is decreased or prevented, and the water otherwise immobilized within the tlocs is added
Admixtures and Special Uses of Cements
355
to that in which the particles can niove. With superplusticizcrs. this can be seen with the light microscope (D44,R32), or in greater detail by SEM of rapidly frozen samples (U14) (Section 8.1.3). With some water reducers. retardation of hydration and increased air entrainment may contribute to the increase in fluidity at any given age, though with lignosulphonates. the fluidity is increased even if air entrainment is avoided through addition o f tributyl phosphate (B139). and superplasticizers in normal concentrations d o not entrain air. Adsorption of the admixture on the hydrating cement grains could decrease flocculation in at least three ways (D44). The lirst is an increase in the magnitude of the (-potential; if all the particles carry a surfi~cecharge of the same sign and sufficient magnitude, they will repel each other. The second is an increase in solid--liquid aflinity; if the particles are more stronglj, attracted to the liquid than to each other, they will tend to disperse. The third is steric hindrance; the oriented adsorption of a non-ionic polymer can weaken the attraction between solid particles. All water reducers, including superplasticizers, appear to contain more than one polar group in the molecule, and many are polymeric. Some simple compounds that are good retarders, such as glucose, are not water reducers. Kondo 6.t (11. (K56) considered that polyelectrolytes were adsorbed on the solid surfaces in the ways shown in Fig. 11.4, so that some of their polar groups bind them to the solid, while others point towards the solution.
Fig. 11.4 Possible modcs of attachment ofpolyelcctrolytc anions to the surface of u particle ofcement: ( A ) train and ( B ) loop modes. Circles rcprcscnt ncgatiwly charged groups. After Kondo r / (11. (K56).
11.4.4 Zeta potential, rheology and nature of t h e sorbent phases Ernsberger and France (E7) showed that addition of calcium lignosulphonate causes cement grains to develop a negative (-potential. Dairnon and
356
Cement Chemistry
Roy (D44.D45) studied the action of superplasticizers by determinations of (-potential and quantities adsorbed. which they related to the flow beha\.iour. In the absence of admixture. the charges on the grains \vex too small to permit reliable determinations of tlie <-potential. but with increasing contents of admixture the latter became increasingly negative. and tended towards a limit of - 30 to -40 mV. Up to about 1% of S N F could be adsorbed, and the increase in [-potential, amount adsorbed. dispersion of the grains and increase in tli~iditywere all positively correlated. Dainion and Roy concluded that the increase in <-potential was the major cause o f t h c improved dispersion. Further rlieological studies (A25.R53) showed that superplasticizers decrease both the plastic viscosi~yand the yield stress. The major eKect was on tlie yield stress, which could decrease almost to m o . Zelwer (ZI I ) found that, in the absence of admixtures, C,S and cement developed weak negative (-potentials. That of C,A was diflicult to study due to Ilocculation, but appeared to be positive, becoming negative on hydration. Kondo c1t (11. (K56) considered that too high n molecular weight was undesirable, as the molecules c o ~ ~ lthen d form bridges between adjacent particles. In contrast, Anderson or (11. ( A X ) found that for sulphonated polystyrenes, the highest (-potentials were obtained with material of high molecular weight, although this was not as strongly adsorbed as that of lower molecular weight. They concluded that the high molecular weight malerial was more likely to show loop as opposed to train adsorption (Fig. S place more of its negative charges into the difYuse double 11.4). and ~ I ~ L Ito layer. Thc fiict that both conventional water reducers and superplasticixers arc more ~Kcctiveif added some time after mixing provides a strong indication that adsorption probably occurs at least in part on the hydrated phases. as rhc anhydrous surfaces have by that time become covered with hydration psoducts. Chioccliio 01 (11. (C57) found that the optimum time for addition \vas at the start of the induction period. More of the admixture seems to be [uken up by the early hydration products, especially of the aluminate phase. i f i t is added before the early reaction has subsided. Costa ct (11. (C58) showed that superplasticizers increase the fluidity of C,S pastes much as they d o that o f cement pastes. Studies on individual anhydrous and hydrated compounds in aqueous and non-aqueous media indicate that calcium lignosulphonate and superplasticizers are adsorbed by C--S~-H,AFm phases or C H but not by C,S, C,A or C,AH, (R54, R55.R56.C5X.M105), though they appear to be taken up by unhydrated (3-C2S (C59). The admixtures also enter interlayer sites of C,AH, and perhaps also of C~-S---H (R.55). intercalation of organic niolecules in C,AH, is a well-established effect (Section 6.1.1 ).
Admixtures and Special Uses of Cements
357
Banfill (B140) noted that the amounts of superplasticizers taken up b) cement were sufficient to form a layer on the anhydrous grains some 60 nm thick. He concluded that multilayer adsorption occurred and that steric hindrance was the major effect. This argument is weakened by the evidence that uptake is largely by the hydration products and that significant amounts of material are absorbed as well as adsorbed. The bulk of the evidence indicates that increase in (-potential is the major effect.
11.4.5
Reasons for the enhanced dispersing power of superplasticizers
As noted earlier, the difference between superplasticizers and conventional water reducers probably lies in the weaker ability of the former to act as retarders, air-entraining agents, or causes of quick setting, which allows them to be used in higher concentrations. The low air-entraining ability can reasonably be attributed to the repeating pattern of polar groups. which provides the molecule with no suitable hydrophobic regions (K56). The reason for the weakness of the retarding power is less obvious. Weak retarding power implies that the hydration products can grow despite the presence of the sorbed material. This might happen because the latter can be assimilated into them. or because the individual bonds between sorbent and sorbate are sufliciently weak that the latter can be displaced by ions adding to the product. Equilibrium with the solution would ensure that the sorbate was readsorbed on the added material.
11.5 Inorganic accelerators and retarders 11.5.1 Accelerators of setting and hardening
Calcium chloride has long been known to accelerate both the setting and hardening of Portland cement concrete. Typically, for concrete cured at 20•‹C, 2% of CaCI,.2H20 by weight of cement might shorten the time of initial set from 3 h to I h and double the I-day compressive strensth. The effect on strength decreases with time, and the final strength can be reduced. Some properties related to the microstructure of the paste, such as resistance to sulphate attack, are also adversely affected. The accelerating effects are greater at low temperatures. They increase with the amount of CaClz used. and at 4% very rapid setting may occur; 2% is probably a reasonable limit. This gives a concentration of about 0.3 mol I - ' in the mixing water for w/c = 0.5. Calcium chloride is typically added as CaC12.2H,0. Chloride ion pro-
358
Cement Chemistrv
motes corrosion of reinforcement, and in many countries, including the U K , CaCI, is no longer used in reinforced concrete. A number of other compounds, such as calcium formate, have been found to have similar properties to CaCI,, but all are both more expensive and less effective.
11.5.2
Mode of action
The addition of CaCI, increases heat evolution at early ages. Supported by other evidence, considered shortly, this indicates that the effects on setting and hardening are due to acceleration of the hydration reactions. Early studies, summarized by Murakami and Tanaka ( M 106). showed that u p e a t many salts have effects qualitatively similar to those of CaCI,. With some reservations, these effects occur with C,S pastes as well as with cement pastes, and are thus seen to be due to faster hydration of the nlite. Fig. 11.5 shows heat evolution curves, obtained by conduction calorimetry, for a C,S paste with and without addition of CaCI,. When the latter is present, the main heat evolution peak begins earlier and rises and falls more steeply; its maximum is reached earlier. The rate of heat evolution at the maximum is positively correlated with the reciprocal of the time at which the maximum occurs (Dl 8). The linear relation extends to organic retarders. As these probably act by hindering the growth of C-S H (Section 11.2.2). this evidence suggests that the accelerators act by promoting it.
TIME ( h ) Fig. 11.5 Rates of heat output from C,S pastes hydrated at 25 C and w/c ratio 0.6, with and wlthout CaCI, in a concentration of 0.0204mol 1 ' . After Brown or trl. (B64).
Admiwtures and Special Uses o f Cements
359
Many studies have shown that both cations and anions contribute to the accelerating effect and that an approximate rank order can be established for each type of ion (E8,M 106,C60,K57,K58,DI8,W31). For the present we shall exclude from consideration such ions as MgZ+ or F - for which precipitation of additional phases complicates the situation. The sequences found by different investigators vary somewhat, and are also influenced by concentration and possibly by the nature of the counter ion. Kantro's ( K 5 7 ) results for the cumulative heat evolution at 12 h for a 3% addition of CaC1,.2H20 and equivalent concentrations of the other chlorides or calcium salts are reasonably typical:
While there are some anomalies, the effect tends to increase with increasing charge and decreasing size of the ion. C a 2 + is considerably more elt'ective than any other cation, suggesting that a specific effect is superimposed on a general one. At very high concentrations, some salts that accelerate in lower concentrations, such as NaCI, act as retarders. At normal concentrations (e.g. 6 1 niol I - ' ) , the accelerating eft'ect of ions is suficiently widespread as to suggest that some general property of electrolytes is involved. The ranking sequences are broadly similar to the Hoffmeister series, which relates to the flocculation of hydrophilic colloids; flocculation is promoted by the action of ions that compete for the H,O molecules that otherwise keep the particles apart. A similar effect may influence the rate at which complexes can be transferred from the solution to the growing particles of C-S-H. Anomalies in relations to size and charge might be explained through differing degrees of solvation or complexation. I t would indeed be surprising if the rate at which the C X H grows was not influenced by the nature and concentration of ions in the surrounding solution, and if at least one of the ions entering the solid did not have a specific and positive effect. The hydration of hemihydrate or anhydrite shows interesting similarities to and differences from that of C3S (G49). Many foreign ions accelerate or retard hydration, but the sequences of effectiveness differ markedly from those found for C,S. Excess SO:-, but not excess of C a Z + ,is a particularly strong accelerator. Murat c.t ul. (M107) found that, for anhydrite, the effectiveness of an added cation decreased with its ionic potential. z/r. and considered that the cations mainly affected the rate of nucleation. The relation to z/r is almost the reverse of what is found with C3S hydration. In other approaches to the problem, attempts have been made to relate
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Cement Chemistry
the accelerating effects either to diffusion of ions (K58,D18), the pH of the bulk solution or the solubility of CH. CaCI, and other salts decrease the pH of the solution (L52,M106,B64), but there is no apparent correlation between the magnitude of the decrease and the accelerating effect (M 106) and NaOH has no significant effect on the rate of C,S hydration in the stage of the reaction under consideration (B64,G49). This latter observation also tells against the view (S103) that acceleration is due to increased CH solubility, since the latter is decreased by addition of NaOH.
11.5.3 Effects on the compositions and structures of the hydration products
In any attempt to understand the effects of CaCI, or other accelerators on the structure or properties of the hydration products it is essential to compare pastes at equal degrees of hydration and not at equal ages. Several early studies (e.g. Refs TI2 and R57) indicated that the addition of CaClz affects the Ca/Si ratio of the C-S-H or the degree of condensation of the silicate anions, but later work does not support either conclusion. Determinations of unreacted C,S, CH and combined water confirmed that substantially more C,S reacts during the first day if CaCI, is present. but showed the latter to have no significant effect on either the Ca/Si or the H20/Si ratios of the C-S-H (L19). SEM of fracture surfaces of both young and mature samples shows that the C-S-H morphology is altered (R57.LI9). At early ages, it changes from the fibrillar Type I to the honeycomb-like Type 11 variety. A study using T G and ,'Si NMR showed that admixture of C,S with CdCI,, Pb(NO,), or malic acid had no significant effect on the Ca/Si ratio of the C-S-H or, if equal degrees of hydration were considered, on the ratio of Q , to Q, silicate tetrahedra, and thus on the relative amounts of dimer and larger silicate ions (C60a). CdCI, is an accelerator, but Pb(NO,), and malic acid are retarders. Ramachandran and Feldman (R57) determined helium porosities and densities for cement pastes containing various quantities of CaCI,. They concluded that for a given degree of hydration, the effect of the latter was to Increase the absolute density and thus also the porosity. At non-evaporable water contents of 12-16%, 1•‹/o or 2% additions of CaCl, slightly increased the compressive strength, but 3.5% additions decreased it. The situation of the CI- ion in the hardened paste is important in relation to the possibility of promoting corrosion. A study on the hydration of C,A and C,S in the presence of gypsum and CaCI, showed that C,AGiCI,.IOH,O was formed after all the SO:- had been used up (T53). No phase containing CI- was detected in the mixes with C,S except at a
Admixtures and Special Uses of Cements
361
20Oh addit~onof CaCI2. Ramachandran (R58) concluded from thermal and other evidence that, in cement pastes made with CaCI,, much of the CI- was chernisorbed or present in interlayer spaces, and that about 20% of i t Lvas not leachable by water. In contrast, analysis of pore solutions expressed from 35- or 70-day-old pastes of cement with NaCl additions showed that 85 98% of the CI- was present i n the pore solution (P48). This proportion was substantially lower for microsilica cements.
11.5.4 Precipitation effects; inorganic retarders and setting accelerators
Many salts precipitate additional phases when added to C,S or cement pastes. If the hydroxide of an added cation is less soluble than CH. either it or a basic or conlplex salt is precipitated. Examples of such precipitates are Mg(OH), or AFni phases (K57). Zn(OH), or CaZn,(OH),.2H20 (A27) and basic lead nitrate or sulphate (T54). Precipitation will leave in solution all or part of the added anion and an equivalent amount of Ca". The calcium salt of an added anion, if of sufficiently low solubility, is similarly precipitated: examples are provided by SO;-, C O i - , PO:-, F - , silicate, aluminate and borate. I n this case, the added cation remains in solution with an equivalent amount of O H - . The salts in these categories have widely varying efTects on setting and hardening. Some, such as lead or zinc salts, phosphates or borates. are strong retarders. Others, such as magnesium salts, behave as setting and hardening accelerators in much the same way as ones not precipitating additional phases. Yet others, such as carbonates, have effects ranging from flash set to retardation of set depending on the concentrations in which they are added. Evidently, more than one effect is involved, and these effects can be delicately balanced. Retardation is probably caused by the formation of protective layers over the cement grains; this has been demonstrated for lead (T54) and zinc (A27) salts and carbonates (U24). In the latter case, it was shown that the layer formed at low concentrations, which caused retardation, was more compact than that formed at higher concentrations, when acceleration occurred. The chemistry underlying accelerating effects produced by salts that yield precipitates is not well understood, and more than one mechanism probably operates. In those cases where a cation is precipitated (e.g. with magnesium salts). the CaZ+released and the anion supplied would be expected to behave like the corresponding calcium salt added at the same stage. CaCI, is a less etrective accelerator if added 12 h after mixing than if added immediately (C60). perhaps because by this time growth of C-S-H is becoming increasingly unimportant as a factor controlling the rate of reaction (Section 7.7.2);
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Cement Chemistry
this niay be expected to reduce the ell'ectiveness of the liberated Ca". The accelerating effect of carbonates in suitable concentrations appears to be confined to the initial stage of reaction. I t occurs with alite as well as with cement (U24), and is thus associated with the behaviour of that phase. I t may be due to increased permeability of a protective layer, as suggested above. but might also be due to removal of C a Z +from the solution. as has been postulated for oxalic acid (Section 1 1.2.4). Admixtures that accelerate the early reaction are used in shotcreting or gunniting, in which a concrete mix is sprayed from a nozzle onto a vertical or steeply sloping surface, and for other applications, such as plugging leaks against pressure. in which rapid setting is essential. Those comnionly used include sodium carbonate, silicate and aluniinate. Setting can thus be caused to begin within minutes, or almost immediately, but subsequent hardening may be delayed and final strengths greatly reduced.
11.6
Effects of high or low temperatures at atmospheric pressure
11.6.1 Hydration at 25-1 00•‹C The hydration chemistry of Portland cement at 25-100•‹C does not difrer essentially from that a t lower temperatures, but the reactions are accelerated and there are some differences in the microstructures and, at temperatures approaching 100-C, in the phase composition of the products. Early work suggested that strengths could be predicted by the use of maturity functions. in which curing time and temperature were regarded as equivalent. In one such approach. the compressive strength of a given concrete mix is considered to be a function of the product of the curing time and T + 10. where T is the temperature in "C (B141). This gives a general indication of the effect of temperature, but such expressions can only be of limited validity. and there is no general agreement on their utility. In low-pressure steam curing, concrete is cured in saturated steam at atmospheric pressure. This is used, especially in cold climates. for the rapid production of precast units. Several reviews exist (B142,B143.112). Various cycles are employed, including ones in which the steam treatment is followed by a period of moist curing at ordinary temperature. In all cases. i t is necessary for satisfactory strength development to cure for several hours at ordinary temperature before raising the temperature and to restrict the rate of temperature increase to 10-20 deg C h - ' . The rate of cooling is probably less critical, but some restriction is generally considered necessary to reduce
Admixtures and Special Uses of Cements
363
thermal stresses. The rates of increase of strength and elasticity increase with temperature. but the final values decrease, especially above about 70 C.
11.6.2
Effects o n kinetics and o n the ultimate extent of hydration
Several studies have shown that the content of non-evaporable water and the chemical shrinkage follow similar trends to the compressive strength; they increase more quickly at first, but the ultimate values are lower ( 1 12.S104). The inference that the hydration reactions d o not proceed as far as at lower temperatures is probably correct, but caution is needed because it depends on the assumption that the stoichiometry and compositions of the products are unchanged. QXDA s t ~ ~ d iof e s the hydration of pure C,S or of the alite in cements have given varied results, some (K21,C39,C61,B53) showing no clear evidence that the ultimate degree of hydration depends on the tctnperature and others showing moderate (V5) or large (A28) decreases with temperature. Verbeck and Helmuth (V5) suggested that rapid hydration leads to encapsulation of the cenient grains by a product layer of low porosity, which retards or prevents further hydration. Because the remaining product would necessarily be more porous. the material would be weaker than one of the same w/c ratio and degree of hydration in which the product was uniformly distributed. An SEM examination (A28) did not support this conclusion. but i t is doubtful whether the technique used (examination of fracture surfaces) would have given conclusive information, and no examinations of polished sections appear yet to have been made. Verbeck and Helmuth's hypothesis would explain the importance of precuring and slow heating and the variability in reported X R D results, since the temperature regime during the early period of hydration would be likely to affect the extent to which a protective layer is formed. An X R D examination of a cement paste that had been moist cured at 90•‹Cfor 2 months with no precuring and a rapid heating rate showed considerable quantities of unhydrated aluminate phase and hydrogarnet or C,AH, (T.5). This tends to support Verbeck and Helmuth's conclusion. The reactions of pfa. ggbfs and other pozzolanic o r latent hydraulic materials are accelerated by increasing temperature to a greater extent than those of the clinker phases. With pfa, substantial reaction occurs within 1-2 days at 40'C or above, which may well be reached within a large pour of concrete. Under conditions simulating the temperature regime at a depth of 3 m, and using a composite cement containing 25% of pfa, about 7 kg of pfa had reacted per I00 kg of composite cement in 3 days and I0 kg in I00 days (C43). The increased rate of reaction is also shown by accelerated consumption of CH (H52).
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Cement Chemistry
11.6.3 Effects on hydration products DTA (K32.025) and XRD (A29) studies indicate that cenient pastes cured at temperatures of or approaching 100•‹Ccontain little or no detectable hydrated aluminate phases, the observation of hydrogarnet noted above thus being unusual. Kalousek and Adanis (K32) considered that the AIJ'. SO,2- and other ions were incorporated into a 'Phase X'. which possibly included all the oxide components of the cenient. In pastes cured normally. much of the AFm phase is probably poorly crystalline and intimntely mixed with the C-S-H (Section 7.3.1). This tendency may be increased if hydration takes place more rapidly. When heated above about 50•‹Cin air. synthetic ettringite yields an almost amorphous product. Buck (B144) found that, for a cement paste initially cured for 24 h at 23•‹C.the ettringite was destroyed on subsequent curing in sealed vials at 75-100•‹C. No decomposition product was detected by XRD. and the ettringite was not reformed on prolonged curing in water at room temperature. The decomposition of the ettringite did not appear to af'f'ect the compressive strength or volume stability. This effect may be a further reason for the non-occurrence of detectable crystalline hydrated aluminate phases. The effect of temperature on the Ca/Si ratio of the C ~ ~-H - S has been examined in several investigations on pastes of C,S or [ K , S by calculation from the contents of CH and unreacted starting material (K21.C39.036. A28,B53.025). The results are conflicting, but tend to indicate increases in the Ca/Si ratio with increased temperature. Some of the earlier results show marked variations in Ca/Si ratio with time. but for low degrees of reaction. results obtained by this method are very sensitive to sniall experimental errors. A later study on laboratory-prepared cements (025) showed no variation with time and a significant decrease in the ratio of CH to C,S reacting at 95•‹Cbut not at 75•‹Cor below. I t also indicated a steady decrease in bound water (defined as the ignition loss after drying with acetone and ether) from 35% on the weight of C,S reacting at 25 'C to 25% at 95 C. The decrease in bound water content could thus be due not only to decreased reaction, but also to decreased interlayer water in the C-S--H, and at the higher temperatures to replacement of ettringite by products of lower water content. X-ray microanalyses ( U 12) for the C-S-H of cement pastes gave Ca,/Siratios of 2.03 and 1.90 for pastes cured at 20•‹Cand 4 5 T , respectively. Studies on calcium silicate pastes show that the distribution of silicate anion size is shifted significantly upwards with a rise in curing temperature (Section 5.3.2). XRD gives no definite indication that the C-S-H formed at temperatures up to 100•‹Cis more crystalline than that formed at ordinary temperatures, though the product of the 2-month treatment at 90•‹Cmentioned earlier (T5) contained a little a-C2SH, a crystalline phase which is
Admixtures and Special Uses of Cements
365
structurally unrelated to C-S-H and readily formed at higher temperatures (Section 1 1.7.2). The H,O and N, specific surface areas and the volumes of pores smaller than 25535 nm determined by N, sorption all decrease with curing teniperature, if allowance is made for ditt'ering degrees of hydration (C62.Sl04. A28.B53,020). Bentur ct ul. (B53) also found an increase in the volume of pores larger than about 4 n m , which they estimated from the amount of water lost on drying at 53% RH. They concluded that increase in temperature causes the hydration products to become more compact. Such an eft'ect is readily explained, as one would expect the anlount of interlayer water in C-S-H to decrease with temperature of formation. It is also compatible with Verbeck and Helmuth's (V5) view that the hydration products are less uniformly distributed. Pore size distributions determined by mercury intrusion porosimetry appear to show little or no variation with curing temperature (020.B144). It is not clear whether this can be reconciled with the above observations.
11.6.4
Low temperatures
Mironov (M 108) summarized the results of studies on concreting at subzero temperatures in the USSR. Hydration continues to the extent that liquid water is present; this in turn depends on the ambient conditions and on the pore structure, and is increased by p~ecuringa t ordinary temperature. At - 5 C, with 24-h precuring, hydration continues at a useful rate, but at or below - IO'C, i t is extremely slow. Additions of CaCI, and other salts act both as accelerators and antifreezing agents ( M 108.M 109). These conclusions were subtantially confirmed by Regourd c/ ul. (R59), who examined a concrete containing 9% of CaCI, on the weight of cement that had been mixed with hot water and cast and cured for I year within a mine at - 10 C. The long-term compressive strength and microstructure were not greatly different from those of a similar concrete cured at ordinary temperature. Calciuni aluminate cements behave well at low temperatures (Section 10.1).
11.7 11.7.1
High-pressure steam curing General
The hardening of concrete or other materials based on calcium silicates may be further accelerated by the use of saturated steam under pressure in an
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autoclave. Typically in such processes, a mix containing Portland cement and finely ground quartz. in addition to aggregate, is cured for 2 -4h in saturated steam at 175•‹C(800 kPa approx.). As with steam curing below IOO'C, it is necessary to precure for a few hours at ordinary temperature, and to restrict the rate of heating, and to a lesser extent that of cooling. In this way, concrete can be produced within 24 h that has a compressive strength at least as high as that obtainable in 28 days at ordinary temperature. Autoclave processes are used to produce a wide variety of materials, ranging from dense concretes and fibre-reinforced materials, through aerated concrete with a bulk density of 400-800 kg ni-"0 very low density thermal insulation materials. The cement provides strength to facilitate handling prior to autoclaving, but otherwise serves essentially as a source of CaO and SiO,, and may be partly o r wholly replaced by othcr starting materials of suitable composition. Calcium silicate bricks (P49) arc made from lime and sand, and are pressed into shape when moist and autocla\~ed. The chemistry of the process is essentially similar to that of thc cement-bascd materials. Aerated concretes (A30) are typically made by incorporating aluminium powder. which reacts during the initial curing below 100 C. generating H,. In autoclave processes in general, both natural and waste materials of various kinds may be used as partial sources of the CaO or SiOz or both.
PERCENTAGE OF CEMENT REPLACED Fig. 11.6 Effects of partial rcplaccmcnt of ccmcnt by tinely ground s i l ~ c aon thc comprcssivc strengths of mixes cured normally and in thc autoclavc. Aftcr Mcnzcl
( M I 10).
Admixtures and Special Uses of Cements
367
Early experience in making autoclaved concrete showed that the prescncc of siliceous material of a sufficiently reactive nature was csscntial for attainment of satisfactory strength. Menzel (M 110) showed that the ratio of such material to cement was critical. Fig. 11.6 shows his results for mixes containing Portland cement and f nely ground quartz. The highest strengths were obtained at a quartz/cement weight ratio of about 30: 70. The optimum ratio depends on the nature and fineness of the siliccous matel-hl.
11.7.2
Basic chemistry of autoclave processes
Understanding of the chemistry of autoclave processes is due primarily to the work of Kalousek and co-workers (K32,KY-K62). Above about 150 C. for the time scales of a few hours that are used in practice, two feuturcs of cement hydration chemistry are added to those relevant at lower temperatures. Firstly, the hydration products tend to crystallize; in the absence of reactive silica. C-S-H tends to be replaced by a structurally unrcluted. crystalline phase, a-C,S hydrate. Secondly, the range of siliceous materinls having effective pozzolanic properties is widened, and includes quartz and various other crystalline minerals, if sufticiently finely ground. Menzel's results were thus explained. Addition of a small amount of quartz brings the bulk Ca/Si ratio to 2.0, which is that of u-C2S hydrate. This phase is relatively dense and crystallizes as rectangular tablets; the product is porous and weak. With larger amounts of quartz. pozzolnnic reaction occurs; the CH is consumed, formation of u-C2S hydrate is avoided, and a C-S-H of low Ca/Si ratio is formed. The porosity is reduced. and a strong material results. The optimum addition of quartz is the maximum that can be taken up; further additions act only as a diluent, and the strength decreases. Table I I. I gives crystal data for u-C2S hydrate and other phases. The optimum addition yields C-S-H with a Ca/Si ratio of 0.9-1 .O. C - S H of this composition tends to crystallize to give I. I-nm tobermorite (Section 5.4.1). Opinions have differed as to the extent to which crystallization is desirable (K60). It has been widely found that optimum times and temperatures of autoclaving exist, which if exceeded lead to decreases in strength. As the main effect of extended time o r increased temperature is likely to be an increase in the crystallinity of the binder, this suggest that. from the standpoint of strength, crystallization is either undesirable or some optimum degree of crystallinity exists. The latter view (B114,F36) appears the more probable. It has received direct experimental support in the case of aerated concrete (A30) and can also account for results obtained with a series of denser lime-quartz materials (C63). It is generally agreed that crystallization
Table 11.1
Crystal data for some synthetic, hydrothermally produced phases
Name
I. I-nm tobermoriten
Formula
[Ca,(Si,O,,HZ)]Ca.4H,O
Lattice parameters ci (nm) b (nm) c (nm) Crystal system Space group
0.564 0.368 2.26 Orthorhombic I' z 1 X-ray density (kg m - 3 ) 2380 References Structure M50. H31 Lattice parameters H61
u-Dicalcium silicate hydrate Ca,(HSiO,)(OH) 0.563 0.369 2.28 Orthorhombic I' 1 2400
"Typical data for normal tobermorite. Data relate to pseudocell "Typical data for anomalous tobermorite with A1 and Na substitution. Data relate to pseudocell ' Structure not fully ordered. Card No. 29--373.
0.9476 0.9198 1.0648 Orthorhombic p21212, 8 2723
Admixtures and Special Uses of Cements
369
decreases drying shrinkage and improves resistance to chemical attack (K60,P49,A30). It is doubtful whether XRD can distinguish a mixture of tobermorite and C-S-H from a uniform material of intermediate crystallinity; the situation may lie between these extremes (A30). This question is disc~~ssed further in Section 11.7.4. Crystallization is probably favoured by low bulk density; its extent is apparently minimal in calcium silicate bricks (P49). but considerable in aerated concretes (A30). In cement-silica materials, substantially all the AI,O, appears to enter the C-S-H, which as its Ca/Si ratio decreases can accommodate increasing amounts of tetrahedrally coordinated aluminium (S70). N M R results (K34) support an early conclusion (K62) that 1.1-nn~ tobermorite, too, can accommodate aluminium in tetrahedral sites. Small amounts of hydrogarnet have sometilnes been detected, especially in products made from raw materials high in AI,O,, such as pfa or slag. Minor amounts of tricalcium silicate hydrate Cjaffeite; C,S,H,) have sometimes been detected (A29,K61).
11.7.3 Mechanisms of reaction and equilibria Quartz reacts relatively slowly, and substantial proportions of C-S-H having a Ca/Si ratio of about 1.75 are formed as intermediate products. which gradually react with i t to give C-S-H of lower Ca/Si ratio. This process competes with the formation of a-C,S hydrate. The conditions should be such as to minimize the latter process, since subsequent reaction of u-C2S hydrate with quartz is slow and may not be completed in the time available. Fine grinding of the quartz and adequate mixing with the cement are therefore essential. Several reviews of phases and equilibria in the Ca0-Si0,-H20 system have been given (R60,T21,T55). Over 20 crystalline ternary phases were listed in the 1980 review (T55), and two more have since been reported (G78,S105). Neither I . I-nm tobermorite nor a-C,S hydrate is an equilibrium product in the pure system at 180•‹Cunder saturated steam pressures. the stable ternary phases under these conditions being possibly truscottite (C,S,,H,), gyrolite (C,S,H,), xonotlite (C,S,H), hillebrandite (C,SH) and tricalcium silicate hydrate, according to the composition (T21). Of these phases, only xonotlite forms readily under normal autoclaving conditions from mixtures in which the SiO, is supplied partly as quartz. It is rapidl! formed from such mixtures at 200•‹C, and because of its good thermal stability in air is used in some autoclaved thermal insulation materials. If the SiO, in an autoclaved material is supplied in a highly reactive form, such as diatomaceous earth, the reaction takes a different course; gyrolite, or, at
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Cement Chemistry
higher temperatures, truscottite. is formed as an early product, and I . I-nm tobern~oritc.if formed at all. results from reaction of' CaO with phases of lower Ca/Si ratio (B145).
11.7.4 Characteristics of hydrothermally formed C-S-H and
tobermorite Mitsuda ct ol. (M 1 I I ) described the X R D patterns. morphology. IR spectra. thermal behaviour and other properties of hydrothermal preparations of C S H . The X R D patterns resembled those of C--S-H(I) (Section 5.4.3). TEM showed that all contained crumpled foils, but some also contained fibrous aggregates. The latter gave electron difrraction patterns corresponding to poorly crystalline tobermorite, in some cases probably intergrown ar or near a single layer level with xonotlite or foshagite (C,S,H). These results show that the distinctions between C-S--H(I), tobermorite and other crystalline phases are not sharp; materials of intermediate crystallinity can exist on the nnnornetre scale. 1.1-nm tobermorites differ in their behaviour on heating. All lose their interlayer water at 100-300•‹C. but this may or may nor be accompanied by shrinkage of the layer thickness to 0.93---0.94nm ((379). Specimens that show lattice shrinkage, called normal tobermorites, crystallize as thin. pseudohexagonal plates with (001) cleavage and a tendency to elongation parallel to h. Those that do not, called anomalous tobermorites. tend to form smaller, more elongated crystals with major (001) and minor ( 100) cleavages. Apart from small differences in lattice parameters, which can be obscured by effects of ionic substitutions (H61), the X R D patterns arc identical. The structure of the individual layers in tobernlorites was described in Section 5.4.2. In I .I-nm tobermorites, these layers are stacked in such a way that oxygen a t o m of the bridging tetrahedra (Fig. 5.7) not attached to two silicon atoms can closely approach those of the adjacent layer. A suggestion (T56,M51) that interlayer Si-0-Si links are present in anomalous tobermorites was confirmed by chemical (W32) and '%i NMR (W21.K 16) studies of silicate anion type. The compositions and densities of naturally occurring tobermorites show that the contents of the pseudocell (Table I 1 . I ) always deviate markedly from the idealized formula C,S,H, (M51). There are at least three effects: (1) The silicate chains can have missing tetrahedra, as in C-S--H(I) (Section 5.4.5). This, the sole major effect in unsubstituted normal tobermorites, is compositionally equivalent to loss of the elements of SiO, and thus increases the Ca/Si ratio and lowers the density. In the
Admixtures and Special Uses of Cements
371
highly crystalline natural materials, the limiting composition is probably about [Ca4Si, ,O, ,Hz]Ca.4H,0, where square brackets exclude the interlayer material. ( 2 ) Interlayer Si-0-Si links are formed. This effect is associated with reduction in interlayer calci~mcontent and is probably a replacement of ( 2 0 - + C a Z + )by -0-, and is thus compositionally equivalent to loss of CaO. I t decreases both the Ca/Si ratio and the density. The theoretical limiting composition is [Ca,Si,O, ,H ,]4H20, but may not be realizable. (3) Substitution of Si4+ by AI3+ is balanced by an increase in interlayer c-2 + or introduction of interlayer alkali cations or both. All known naturally occurring anomalous tobermorites show effects (2) and (3). I . l -nm tobermorite is readily synthesized sing CH and fineiy ground quartz at 180•‹C.I t is much less readily formed if amorphous silica is used. Synthetic studies show that normal tobermorite is an intermediate in the transition from C--S--H(I)to anonialous tobermorite (E9,H61). Its forniation in preference to anomalous tobermorite is favoured by short time, low temperature, high Ca/Si ratio, and presence of Al" in the absence of alkali. Thc presence of Al" together with alkali favours the formation of anomalous tobermorite (E9). The presence of alkali without Al" has been variously found to impede crystallization (E9) or to favour formation of anomalous tobermorite (H61). Most of these results are readily explainable in terms of the structural differences described above. Al" is also reported to accelerate the formation of tobermorite and to retard its replacement by sonotlite at temperatures around 175•‹C(K62). Mitsuda and Chan (MI 12) found the tobermorite in some aerated concretes to be anonialous. Aluminium-si~bstitutedtoberniorites have cation exchange properties (K63). Hara and lnoue (H61) studied the lattice parameters of tobermorites. They confirmed an early observation (K62) that c increases with aluniini~m substitution, but found that several other variables affect the parameters to snialler extents. The u-axial lengths of highly crystalline, natural toberniorites appear to be lower (0.560-0.562 nm for the pseudocell) than those of the synthetic materials (M5 1).
11.8 Oil well cementing
11.8.1
General
In oil well cementing (R61,D46,S106) a cement slurry is pumped down the steel casing of the well and up the annular space between it and the
372
Cement Chemistry
surrounding rock. The main objectives are to restrict movement of Ruids between formations at different levels and to support and protect the casing. More specialized operations comprise squeeze cementing, in which the slurry is forced through a hole in the casing into a void or porous rock. and plugging. in which the casing is temporarily or permanently blocked at u specified depth. With current technology, oil wells are typically up to 6000 m deep. Thc temperature of the rock at the bottom of the well ('bottom hole static temperature') at that depth is 100-250•‹C. The maximum temperature of the slurry during pumping ('bottom hole circulating temperature') is in general lower but may still be as high as 180•‹C.The pressure experienced by the slurry during pumping is equal to the hydrostatic load plus the pumping pressure, and may be as much as 150 MPa. The entire depth of a deep well would not be cemented in a single operation, but even so, pumping can take several hours. In geothermal wells, the maximum temperatures encountered may exceed 300•‹C.
11.8.2 Types of cement and of admixtures
The slurry must remain sufficiently mobile for the pumping operation to be completed and must provide adequate strength, resistance to flow of liquid or gas and resistance to chemical attack after it has been placed. By using a high-temperature, high-pressure consistometer, in which the regime of temperature and pressure during pumping is simulated, it is possible to monitor the changes in consistency and to predict the 'thickening time' during which pumping is practicable. The thickening time is akin to final set under the simulated well conditions. The American Petroleum Institute has defined various classes of Portland cement, of which G and H are widells used. The specifications of both are typically met by sulphate-resisting Portland cements, coarsely ground to 280-340 m2 kg-' (Blaine) for Class G or to 200-260 m2 kg-' for Class H (S106,B146). Free lime is minimized to permit good response to admixtures. Both classes are intended to be used as basic cements at depths down to about 2500 m, and at greater depths with suitable admixtures. In practice, admixtures of many kinds are used at most depths. Retarders and dispersants (water reducers) are widely employed, especially in the deeper wells and also to counteract the effects of other admixtures that have incidental accelerating effects. Lignosulphonates, modified lignosulphonates, cellulose derivatives and saturated NaCl are among those used. NaCl is effective up to about 1 30•‹C,and modified lignosulphonates to at least 150•‹C.Superplasticizers may be used up to 150•‹C.Accelerators.
Admixtures and Special Uses of Cements
373
such as CaC1, (2-4% on the weight of cement) o r NaCl (2.0-3.5%) are also used. In marine locations, sea water is often used for mixing, and acts as an accelerator. Sodium chloride may be added to fresh water; in addition to its accelerating or retarding properties, it reduces damage to salt and shale strata and causes the hardened paste to expand. Various admixtures are used to modify physical properties of the slurry, though some have incidental chemical effects that are countered by the use of further admixtures. It may be necessary to adjust the bulk density, e.g. by adding hematite to raise it, or bentonite (sodium montmorillonite) or sodium silicate to lower it. Bentonite greatly increases the water demand, but may be used together with calcium or sodium lignosulphonate and sometimes NaCl to obtain a desired combination of density, fluidity, w/c ratio and thickening time. The optimum addition of untreated bentonite is 8%, but this is reduced to 2% if the bentonite has been prehydrated, i.e. mixed with water and allowed to swell before being used. Granular, lamellar or fibrous materials are occasionally added to prevent slurry from being lost in rock fissures when lightweight or thixotropic slurries are insufficiently effective. Expanded perlite (a heat-treated volcanic material), walnut shells, coal, cellophane and nylon are examples. Nylon or other fibres have been used to increase shear, impact and tensile strength but sometimes present logistical problems. Cellulose derivatives, water reducers and latex admixtures are among admixtures that reduce loss of solution from the slurry into porous strata. More specialized admixtures include radioactive tracers, which may be detected by devices lowered down the hole to trace the movement of the slurry. Dyes or pigments may similarly be used to check its emergence. Some of the chemicals added to drilling muds are strong retarders for cement. Proprietary 'spacer fluids' are commonly pumped ahead of the cement slurry to counteract contamination. Admixtures of paraformaldehyde and sodium chromate are sometimes also used. Addition of 5-10% of gypsum produces a thixotropic slurry, which can be pumped but which gels rapidly when stationary; this can be used to help the slurry to pass permeable formations. Gypsum also causes expansion, and it or other expansive admixtures may be used to improve the seal with the rock o r casing.
11.8.3
Effects of temperature a n d pressure
At the high temperatures encountered in deep wells, pozzolanic admixtures are essential to prevent strength retrogression, as in high-pressure steam curing (Section 11.7). Silica flour (finely ground quartz) and silica sand are the most commonly used. There are few data on the effects of prolonged
374
Cement Chemistry
cxposure of cement-silica mixes to hydrothermal conditions; pastes cured for 2.5-4 months at 140-170•‹C and saturated steam pressures are reported to have similar strengths to those so treated for shorter times, though some replacenlent of tobermorite by gyrolite and xonotlite was observed ( I 13). In ;I deep well, the pressure niay be much above that of saturated steam; this alters the equilibria (R60,T21) and possibly also the kinetics. At 110'C or 300 C and 7-68 MPa, mixtures of cement and quartz were shown to gi\-e C-S-H(I), which began to change into tobermorite within 32--64 h (027). In hot. geothermal wells, many phases can form in addition to or in place of those found in autoclaved cement materials (L53). Special problems arise in cementing wells in the Arctic, where permafrost may exist to a depth of 1000 m. It is necessary that the cement should set at low temperatures, that the surrounding ground should not be disturbed b\, melting or erosion during drilling or cementing or in the subsequent life of the well, and that freezable liquids should not be left in the annular space. Mixtures based on Portland cement and gypsum have been used (S106).
11.9 Very high strength cement-based materials
11.9.1
General
This section deals with cement-based materials having conipressive strengths much above 100 MPa o r comparable uprating of other mechanical properties. Relatively modest improvements yield materials with potential specialist uses in construction. Larger ones, especially in tensile or flexural strength and fracture toughness, offer the possibility of making low-volume, hightechnology materials. The con~pressivestrength can be increased by lowering the w/c ratio. As the latter decreases, the particle size distribution of the starting material beconies increasingly important. Brunauer er ul. (B147) described the preparation and properties of cement pastes with compressive strengths up to 350 MPa. Portland cement clinkers were ground to 600-900 mZ k g ' using grinding aids and subsequently mixed at w/c 0.2 with admixtures of calcium lignosulphonate and K,CO,. Very high strength concretes have since been obtained using Portland cements with superplasticizers and microsilica. In so-called DSP (densified systems containing homogeneously arranged ultrafine particles) materials. the use of low w/s ratios (0.12-0.22), special aggregates, including fibres. and special processing conditions allows compressive strengths of up to 270 MPa to be obtained, with good resistance to abrasion and chemical attack
Admixtures and Special Uses of Cements
375
(H53,H62). The properties were attributed to a combination of erects. The particles of microsilica, being much finer than those of the cement. partially fill the spaces between the cement grains, and this, together with the superplasticizer, allows the latter to pack more uniformly. They also provide nucleation sites for hydration products, undergo pozzolanic reaction and probably improve the paste-aggregate bond. Lime-quartz materials with compressive strengths of up to 250 MPa can be made by moulding the starting materials under a pressure of 138 MPa before autoclaving (T57,C64). Portland cement pastes have similarly been pressure moulded to allow use of w/c ratios down to 0.06 and development of 28-day strengths up to 330 MPa (R62). Roy and co-workers obtained st111 higher strengths by hot pressing Portland (R62,R63) o r calcium aluniinate (G80) cement pastes, followed by normal curing in water. A Portland cement paste pressed for I h at 250•‹C and 345 MPa and then cured for 28 days had a strength of 650 MPa. The compressive strengths and total water porosities (I .8% in the case mentioned) obeyed the Schiller relation (Section 8.4.1 ). Impregnation with an organic polymer of a concrete that has already developed some strength is another way of reducing porosity and increasing strength. The concrete is first dried, and the monomer is then introduced and polymerized in situ by y-irradiation or by including a catalyst and subsequently heating at 70-90•‹C (114). Polymethylmethacrylate (PMMA) is possibly the most effective polymer. For maximum uptake of polymer, it 1s necessary to dry strongly (e.g. by heating at 150•‹C)and to evacuate, but useful amounts can be introduced without evacuation. Compressive strengths can be approximately doubled, and values around 220 MPa have been reported for cement-sand mortars. Vacuum impregnation of cement pastes and autoclaved materials with molten sulphur at 128•‹C similarly increased microhardness and elastic modulus by factors of up to 6 and 4. respectively (F45). The tensile or flexural strengths of the materials described above were. in general, about one-tenth of the compressive strengths, as in normal cement pastes or concretes. In so-called M D F (macro-defect-free) cements, Birchall and co-workers (B148,K64) obtained higher relative values of flexural strength and other mechanical properties. The materials were made by incorporating a water-soluble polymer, such as poly(vinylalcohol/acetate), into a Portland or calcium aluminate cement mix to produce a dough that could be processed by methods of conventional plastic technology, such as pressure moulding, extrusion or calendering. They had flexural strengths of 40-1 50 MPa, Young's moduli of 35-50 GPa, compressive strengths of 100300 MPa and fracture energies of 40-200 Jm-2.
376
Cement Chemistry
11.9.2
Phase compositions, microstructures and causes of high strength
In general, XRD and other studies show that the phase compositions of the very high strength materials described above are similar to those of weaker ones of similar types, the proportion of unreacted clinker phases increasing with decrease in porosity. The major product detected in hot pressed calcium aluminate cements was C,AH, (G80); this is consistent with Feldman and Beaudoin's (F36) view that this and similar dense, crystalline phases produce high strengths if sufficiently closely welded together in materials of low porosity. The same applies to unreacted clinker phases. The microstrutures of hot pressed cements (R62) and M D F cements (S107) are dense and compact, and CH crystals, where present, are smaller and more uniformly distributed than at lower porosities. In M D F cements, CH can occur as lamellae only 10 nm thick (G37). Pastes inpregnated with PMMA or sulphur are still sufficiently permeable to water that expansion occurs on long exposure (F46). In polymerimpregnated (S108) and MDF (R64) cement pastes, there is evidence of interaction between Ca2+ ions and carboxylate and possibly other groups of the polymer. In M D F pastes made with calcium aluminate cement. the polymer (PVA) was found to inhibit the normal hydration reactions of the cement, but to react with Ca2+ and A13+ to give an ionically cross-linked polymer and calcium acetate. TEM showed the material to be essentiall) a dispersion of grains of clinker or hydration products in a continuous polymer matrix. Kendall et al. (K64) considered that M D F cements owed their high strengths primarily to the absence of large flaws, which they held to be the reason for the relative weakness of normal cement pastes. This view has been challenged (Bl l5,K65,E4). Beaudoin and Feldman (B 1 15) discussed ways in which stress concentrations were reduced in very strong cement pastes. and both they and Eden and Bailey (E4) considered that in MDF pastes the polymer plays a major role.
Concrete chemistry 12.1
12.1 .I
Cement paste in concrete Portland cement mortars and concretes
Concrete cannot properly be described as a composite of coarse and fine aggregate in a matrix of cement paste otherwise identical with the aggregate free material. The microstructure of the paste close to the aggregate differs from that of cement paste in bulk, and much of the paste in a concrete or mortar is in this category (D47). Because the strength of concrete depends in part on that of the cement-aggregate bond, many of the studies on concrete microstructure have concentrated on this aspect. Several reviews esist (S109,M 1 13.D48,M 1 14). Using light microscopy to examine thin sections of mortars impregnated with a coloured resin, Farran (F43) demonstrated the presence in some cases of an interfacial zone or aureole of increased porosity and presun~ably lower strength. Microhardness measurements confirmed this (L54). Farran er a/. (F47) studied double replicas of abraded surfaces of mortars by TEM. and thereby confirmed the existence of interfacial zones up to some 35 pm wide and of distinctive microstructure. Javelas et 01. (534) examined ionthinned sections of mortars by TEM. The surfaces of calcite or quartz grains in mortars were closely covered with layers of poorly crystalline material. which gave a diffuse diffraction ring with a spacing of 0.3 nm. This could not have been due to CH, and the material was probably C-S-H. Several studies have been made on specimens prepared by casting cement paste against relatively large, flat and often polished surfaces of aggregate and then split apart near to the interface. Barnes et 01. (B149) made SEM examinations of the interfacial zone in pastes cast against glass slides. They reported the occurrence of duplex films, comprising a layer of CH crystals in contact with the glass and oriented with their c-axes normal to its surface, and a layer of C-S-H further out into the paste. The entire duplex film \\as typically 1 .O-1.5 pm thick, and formed part of a much thicker interfacial zone, prominent features of which included hollow-shell grains and CH crystals with c-axes both parallel and perpendicular to the glass surface. They observed similar effects in fracture surfaces of mortars ( B 150). Grandet and Ollivier (G81) made X R D examinations of layers of paste at
378
Cement Chemistry
difrerent distances away from various aggregates. exposed by progressive abrasion. CH close to the interface showed strong preferred orientation. with c normal to the interface; the degree of orientation dccrcnscci \\.it11 the distance, becoming zero at about 40pm. Monteiro c l / (11. (M 115) and Detwiler ct LII.(D49) confirmed this observation, which dilkrs horn those of Barnes or 01. in showing no tendency to preferred orientation with ( , parallel to the interface. The growth of CH crystals with c normal to the interface has been described as epitactic, but this is incorrect, as there is no preferred crystallographic orientation relative to the crystal structure of the substrate. which can indeed be amorphous. Struble and Mindess (SI 10) observed n layer of well-oriented CH adjacent to the aggregate surface. but no duplex film, and noted that the interfacial zone appeared to be of higher c ratio than the bulk paste. Wu et (11. (W33) and Monteiro and co-workers (M 116.M 115) observed that, within the interfacial zone, the content of ettringite increases as the aggregate surface is approached.
12.1.2.
Backscattered electron imaging of t h e interfacial zone
Studies of prepared surfaces have yielded interesting information. but their limitations are becoming increasingly apparent. As Scrivener a n d Gartncr (S1 I I) have noted, the aggregate surfaces in a real concrete or mortar are not polished, flat and widely separated, but in varying degrees irregular and close together. During mixing, the aggregate particles are in vigorous motion relative to the surrounding paste, and during setting, localized bleeding below aggregate particles o r reinforcement can occur. S t ~ ~ d i eofs fracture surfaces suffer from the further serious limitations that they are not representative of the bulk material, but of its weaker regions, and that X-ray microanalyses on them are a t best only semiquantitative. The high resolution provided by secondary electron images aggravates the problem of obtaining statistically representative information. Backscattered electron images (BEI) of polished sections (SI I I-SI 14,S68) d o not suffer from these limitations, and are readily combined with quantitative X-ray microanalysis and with image analysis to provide further types of quantitati\.e information. From an SEM study of mortars using both fracture surfaces and BE1 of polished sections, Scrivener and Pratt (SI 12) concluded that a layer of C-SH about 0.5 pm thick was formed over the surfaces of the sand grains during the first day. This layer resembled that formed on pfa particles. They confirmed the tendency to increased content of ettringite in the interfacial region. Large crystals of CH were sometimes formed adjacent to the sand
Concrete Chemistry
379
grains, and tended to predominate on fracture surfaces because of their good cleavage; their orientation relative to the aggregate surface varied consideritbly. Particularly large crystals or extensive deposits of C H were found in places where local bleeding was considered to have been likely. Regions poorer in cement particles sometimes occurred on one side only of a sand grain and possibly arose from the sweeping action of the latter during mixing. In any attempt to estimate distances within an interfacial zone from BE1 of polished sections, allowance must be made for the fact that the interfaces examined are in general not perpendicular to the section. This problem may be handled by examining a sufficient number of grains (SI 14) or, less satisfactorily, by examining specimens in which the paste has been vibrated in contact with a single piece of aggregate with one surface in a known orientation (SI 11). Studies by BE1 with image analysis by the former method showed that as the aggregate surface is approached within the interfacial zone, the average content of unreacted clinker phases decreases and the porosity increases (Fig 12.1). The average C H content, in contrast. appears to show only a slight increase (S111). The region within about I0 pm from the surface was found to be deficient in the usual relicts of large clinker grains, indicating that it is difficult for the latter to pack close to the interface. The grains smaller than about 5 pm had often hydrated completely, giving hollow-shell grains, which were thus relatively abundant in the interfacial region. These results gave no indication of the presence either of duplex films or of continuous layers of C H at the aggregate interfaces in normal concretes or mortars. The phase most often in contact with the aggregate surface appears to be C-S-H; this is consistent with the early results of Javelas ct u/. (534). Relatively large crystals or polycrystalline masses of C H may occur locally close to the aggregate surface, but in some cases at least, this results from major increases in local porosity caused by the sweeping action of the aggregate particle during mixing or from bleeding. Observations of a high concentration of CH at the interface in fracture surfaces may also result from preferred fracture through weaker regions in which this phase is present (S 1 1 5 ) .
12.1.3 The nature of t h e paste-aggregate bond
The attractive forces between paste and aggregate are in principle likely to be similar to those existing within the paste (Section 8.4.3). Chemical reactions between paste and aggregate can strengthen the bond by corroding the aggregate surface and thereby increasing the area of contact (F43,G82), but
380
Cement Chemistry
DISTANCE FROM INTERFACE p m
30
I
(b) POROSITY
10
30 40 50 60 DISTANCE FROM INTERFACE hrn 20
70
Fig. 12.1 Microstructural gradients in the interfacial region of a concrete: (a) unreacted clinker phases, (b) porosity, determined using SEM with image analysis. Each point represents a mean from 50 determinations, and the standard errors were approximately 10% relative. Scrivener el al. (S114).
Concrete Chemistry
381
they can also weaken the material by creating expansive forces too strong for i t to withstand. Struble ct crl. (S109) reviewed early work on the pasteeaggregate bond. Postulated superficial pozzolanic reactions at ordinary temperatures between paste and quartz or other coninion siliceous aggregates appear to be unsupported by experiniental evidence, except for the deleterious alkalikilica reaction (Section 12.4). On the other hand, there is considerable evidence that superficial reaction occurs with calcite aggregate and that it strengthens the bond with the paste (F43,B151,G82,M117). From X R D and SEM studies on composites of pastes cast against marble surfaces and on pastes made from cement mixed with finely ground calcite, Grandet and Ollivier (G82) concluded that c,AC,,,H,, was formed initially and that it was later replaced by C,ACH, . The calcite surfaces were strongly pitted. This reaction. earlier postulated by Lyubimova and Pinus (L54), was noted in Section 9.6.3 in connection with the use of calcite a s a filler. Monteiro and Mehta ( M 117) found that the calcite was similarly pitted if Lilite was substituted for cement and reported the appearance of an X R D peak at 0.79 nm. They considered that the reaction product, either with alite or with cement, was not a C0,2--containing AFm phase but a basic calcium carbonate. Such phases have been reported as synthetic products (SI 16) and as a natural mineral. defernite (P50).
,
12.1.4.
Composite cements and other topics
Partial replacement of cement by pfa can either increase or decrease the width of the interfacial zone, depending on the characteristics of the pfa; slag has relatively little effect (C65). In concretes made with microsilica and superplasticizers, the paste is dense right up to the aggregate surfaces (R65.B 152,s l l I). Scrivener and Gartner (S 1 1 1) found some increase in porosity, but this could be almost entirely attributed to the presence of hollow-shell grains. With pastes vibrated against a single piece of aggregate. in contrast. the replacement had no significant effect on the distribution of porosity within the interfacial zone, and some clumping of the microsilica was observed. Clumping was found to occur in pastes even when a superplasticizer was used (Sl 15), suggesting that the coarse aggregate breaks up the clumps by crushing or shearing during mixing. For a given w/s ratio. partial replacement of cement by microsilica increases the strength in concrete but not in pastes (B152). It also increases the calculated porosity (R66) (Section 9.7.1), and the strength increase in concrete of a given wjs ratio is thus due to the improvement in the paste-aggregate bond (R66,B152,Sl l I).
382
Cement Chemistrv
Interfaces of calcium aluminate cements with aggregates were discussed in Chapter 10. The interfacial zone between Portland cement paste and stccl has bcen studied by SEM, using various techniques of specimcn preparation (e.g. A31,B153). The major features observed have been substanri:tl deposits of CH and, further away from the interFdce, a relatively weak. porous zone. Bentur 01. (B135) also reported the occurrence of a duplex lilm. Interfaces of cement or C,S pastes with zinc (A32,T58) or copper (T58) have bcen studied.
12.1.5 Effects at exposed surfaces The microstructures of the surface regions of a concrete dill'er from that of the bulk material, and are important because they have a major effect on durability. The differences have several origins. Settlement and constraints imposed by the formwork may affect aggregate distribution. Moisture gradients can be created if a surface is exposed to a drying atmosphere. retarding or even stopping hydration in the surface layers, altering the pore structure and facilitating carbonation. Water can be lost by absorption in or leakage through formwork. Patel ct cil. (P5 I ) moist-cured blocks of cement pastes of w/c ratio 0.59 for 7 days and then sealed the prism surfaces of each and exposed the ends to air at 20•‹Cand 65% R H . Methanol sorption data obtained using porous glass as a reference standard (Section 8.3.5) showed that, near to the exposed surfaces, the pore structure was markedly coarser and the dill'usion time lower. T G evidence showed that less hydration had o c c ~ ~ r r eand d that carbonation was increased. These etl'ects were detectable even at a distance of 50mm from the exposed surface. Further work confirmed the marked effect of RH on hydration rates (P28) (Section 7.7.1). Crumbie ct (11. (C66) moist-cured concrete specimens with w/c 0.4 and 0.6 in steel moulds for 24 h and then cured them in saturated C H solution for 18 days. Slices from the top, bottom, sides and bulk of each specimen were examined by methanol exchange. quantitative determination of coarse and fine aggregate and backscattered electron imaging in the SEM. The largest variations in the aggregate distribution were in the contents of coarse aggregate in the top 5 mm, which were much reduced, especially in the niis with w/c 0.6, for which the content was almost zero. The methanol exchange data showed that the porosity and permeability increased near to all the surfaces. SEM examination of a section cut perpendicular to the top surface confirmed the increase in porosity, which was about twice as high in the outermost 200 pm as in the bulk. The surface region was also depleted in CH, but the content of anhydrous material was little affected. Despite the
Concrete Chemistry
383
fact that the specimens had been placed in saturated CH, some leaching appeared to have occurred.
12.2
Durability-general
aspects
If it is properly designed and produced for the environment in which it has to serve, concrete is an extremely durable material. If the design or production is inadequate, it may deteriorate. Sonie environments exist in which no concrete is durable. Deterioration may occur through a variety of chemical or physical processes. The rest of this chapter deals with these, with emphasis on chemical processes in which the cement paste is involved. For convenience, the various forms of attack are considered separately, but examination of deteriorated concretes often shows that more than one form of attack has been operative. This is often because one form of attack renders the concrete more susceptible to damage by another. Portland and composite cement concretes are considered; calcium aluminate cements were discussed in Section 10.1. I 1. The concept of a design life for concrete structures is becoming increasingly recognized (SI 17), failure then being defined as deterioration of properties to an unacceptable level at an age lower than the design life. To design a concrete for a specified minimum life, it is necessary to understand the processes that cause deterioration, including the rates at which these will occur under the conditions to which i t will be subjected. In this last respect. a wide range of climatic, chemical and physical factors must be considered. 12.3 Carbonation, chloride penetration and corrosion of reinforcement
12.3.1
General
Corrosion of reinforcement is probably the most widespread cause of deterioration in concrete. The expansion produced by rust formation causes the surrounding concrete to crack and spall. In a sound concrete, rusting is prevented by the high pH of the pore solution, which stabilizes an oxide film on the steel that inhibits further attack. This film is unstable at lower pH values, which can result from carbonation or leaching, or in the presence of CI-. Sources of the latter include sea water or salt spray, de-icing salts used on roads, certain aggregates, especially those available in desert climates. and CaCI, used as an accelerator. Though now widely prohibited, this can
384
Cement Chemistrv
still affect older structures. Tensile stress can damagc the protective film. and particular care must be taken to minimize ingress of COz and CI- in prestressed structures. Carbonation begins at exposed surfaces. and spreads inwards at n rate proportional to the square root of time (K66). The thickness of the afl'ected Inyer can be approximately determined by testing a section with phenolphthalein. Chloride ion froni an external source similarly penetrates inwards. The age at which corrosion is liable to begin thus depends on the mininiuni thickness of concrete covering the reinforcement, the resistance to penetration and the conditions to which the concrete is subjected. The depth of cover needed to provide protection over a given period can be calculated ( SI 17,B154). The resistance to penetration depends on the permeability of the concrete, which is affected by cement content. w/c ratio, aggregate grading. degree of compaction and adequacy of curing. Lack of bonding between the steel and concrete decreases this resistance and also destroys the protection afforded by the alkaline pore solution. Microcracks are in some circumstances closed up by the products of carbonation or by ingress of dust, but this may not occur. especially under fluctuating loads. which may thus accelerate the onset of corrosion. With a dense, well-made concrete. 15 mm of cover may suftice to provide virtually indefinite protection, but with a porous or poorly made concrete, it may allow corrosion to begin within a few years (B154). lniportant environmental factors. in addition to stress and availability of COz, 0, and C1-, include RH and temperature. At low hunlidities. gases penetrate easily through empty pores, but lack of water may inhibit both carbonation and corrosion; at very high humidities, these efyects are reversed. Chloride penetration requires the presence of water. Attack is thus most serious at intermediate humidities. Roy (R67) reviewed carbonation and transport and reactions of chlorides. Hansson (H63) and Turriziani (T59) reviewed corrosion.
12.3.2
Carbonation
Carbon dioxide dissolves in the pore solution of cement paste, producing C0:-, which reacts with C a 2 +to produce CaCO,. The O H - and C a Z +ions required by these reactions are obtained by the dissolution of CH and decomposition of the hydrated silicate and aluminate phases. C,AH, is quickly converted into c,ACH,, and ultimately into CaCO, and hydrous alumina; monosulphate and ettringite yield CaCO,, hydrous alumina and gypsum. C-S-H is decalcified, initially by lowering of its Ca/Si ratio. and ultimately by conversion into a highly porous, hydrous form of silica. The
Concrete Chern~stry 385
reactions involving the CH and C S H may be represented by the Sollowing equations: COz + 2 0 H - + C O : Ca'
+
+CO:
Ca(OH), .uCaO.SiO,(aq)
+ zH,O
-+
4
4
+ HzO
( 12.2)
CaCO,
Caz+ + 20H
y C a 2 + + 2j9OH -
(12.1)
-
( 13.3)
+ (-1--!*)CaO.SiO,(aq) ( 13.4)
SEM by backscattered electron imaging shows that, in the c a r 1 stage of carbonation. a dense rim consisting largely of CaCO, is formed at the surface (Fig. 12.2). Within this zone, C H is largely absent 2nd C S H. including that formed in situ from the alite or belite, is in varying degrees decalcified. The CaCO, can form relatively large, continuous masses. The microstructural aspects of the reaction have not been systematically s t ~ d i e d . but it appears likely that extensive reorganization occurs.
Fig. 12.2 SEM (backscattercd clcctron image) of a concrctc showing a carbonated surfacc layer 50-100 pm thick, forming a dcnsc rim within which C-S-H h x bccn decalcified to give a silica gel low in CaO. Bright rcgions arc of itnhydrated clinker and dark. angular regions arc of aggrcgatc. Courtesy W H D Microanalysis Consititants Ltd., Ipswich, U K (H64).
The results of the phenolphthalein test show that the pH falls to 8.5 or below in the carbonated zone, but no more detailed studies on the composition of the pore solution have been reported. Andrade c.t (11. (A33) concluded that a value as low as 8.5 could only be explained if alkali cations were removed from the solution by leaching or uptake by the hydrous silica
386
Cement Chemistrv
formed by decomposition of the C-S-H. In the pure (K.Na),O CO, H 2 0 system, a pH of 8.5 corresponds approximately to formation of HCOI. Suzuki p t (11. (SI 18) studied the mechanism of carbonation of C- S H in suspensions. Barret ct (11. (B122) discussed equilibria relevant to the carbonation of C,AH,. Apart from the effect on reinforcement corrosion, carbonation has both good and bad effects on concrete. I t markedly increases the compressive and tensile strengths of Portland cement mortars and concretes. sometimes by as much as loo%, in the regions affected. The strengths also increase if the binder is a composite cement made with 40% of slag. but decrease if this proportion is increased or with supersulphated cements (MIIX). The changes in strength are paralleled by ones in weight for a specified drying condition, indicating that gain in strength occurs if the uptake of CO? exceeds the loss of bound water. Carbonation also makes a specific and substantial contribution to irreversible shrinkage; as Lea (L6) pointed out. this indicates that the C-S-H is attacked. I t may cause superficial crazing. and can contribute significantly to the drying shrinkage measurements on small specimens. The reduction in permeability due to carbonation of the surface layers results in a decrease in the reversible volume changes with subsequent variations in RH.
12.3.3 Transport and reactions of chlorides
The risk of chloride-induced corrosion increases with ease of migration of C1- through the cement paste and with the ratio of C1- to OH- in the pore solution (P52). The diffusion of CI- through cement pastes was discussed in Section 8.6.2. In concrete, transport can also occur through regions of enhanced permeability or microcracks and diffusion is supplemented by capillary action except in a completely saturated material. Factors afrecting the CI-/OH- ratio include the O H - concentration and the extent to \ihich CI- is taken up by solid phases. The O H - concentration depends on the alkali content of the cement and will often be lower for a composite than for a pure Portland cement. Chloride is partly bound in C,ACaCI,.IOH20 but much remains in the pore solution (P48,M 119,R67,R68). With very concentrated chloride solutions, other solid phases may be formed (Section 12.6.3). Formation of C,ACaCl,.IOH,O may be decreased in the presence of sulphates,:due to preferential formation of monosulphate or ettringite. or by carbonation, and possibly depends on the source of the CI- (P52). Page er al. (P52) determined the CI-/OH- ratios in the pore solutions. C1- diffusivities and corrosion rates of embedded steel in pastes of cements of several types. With Portland cements, the CI-/OH- ratios decreased
Concrete Chern~stry 387
with potential C,A content; cements containing 30'%, pfa or 65'lb ggbfs gave lower ratios than any of the Portland cements. The O H - concentrations were lower for the composite cements, and the low CI-/OH- ratios were presumably due to greater uptake of CI- by the hydration products. The C1- diffusivities increased in the sequence slag cement < pfii cement < ordinary Portland cements < sulphate-resisting Portland cement. In conformity with these results, the corrosion rates were slightly lower for thc composite cements than for an ordinary Portland cement, and considerably lower than that for the sulphate-resisting Portland cement. The authors stressed the need to study a wider range of cements and chloride solutions before any general conclusions could be made. The results nevertheless indicate that binding of CI-, as well as diffusivity, plays an important part in limiting chloride-induced corrosion. A microstructural study (M120) showed greater corrosion with a microsilica cement than with a pure Portland cement; replacement of cement by microsilica may be expected both to lower the O H - concentration and to decrease the content of AFm phase able to take up CI-.
12.3.4 Corrosion
The corrosion of steel in concrete is an electrochemical process. Due to local compositional or structural variations, some areas become positilely and others negatively charged, and electrical cells are set up. The anodic reaction and the principal cathodic reaction are respectively typified by the equations 3Fe
+ SOH-
and 4e-
-+
Fe,O, + 4 H 2 0 + 8e-
+ 2 H 2 0 + 0, + 4 0 H -
( 12.5) ( 12.6)
At the anodes, iron is dissolved and an oxide deposited. Electrons travel from anode to cathode within the metal, and O H - ions travel from cathode to anode through the solution with which it is in contact. For these processes to occur, a continuing source of oxygen is needed, and the surface of the metal must remain wet. If pH is above about 11.5 (P48) and CI- is absent, the oxide is deposited as a thin protective film which is virtually continuous, and the rate of attack is so low as to be insignificant. The iron is said to be in a passive condition. At a lower pH, an oxide or oxyhydroxide is deposited in an incoherent form, and corrosion is rapid. Chloride ions cause local breakdown of the passive film, even at high pH. The regions of metal thus exposed become anodes and the unaffected areas
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Cement Chemistry
become cathodes. Since the areas of breakdown are small. high current densities can develop at the anodes, causing pitting and localized decrease in pH. which aggravates the attack. High local ratios of CI- to O H - in the solution favour pitting (P52). As was noted in Section 12.3.1, corrosion may be inhibited if the permeability of the concrete to water or oxygen is low enough. and thus does not necessarily occur even if the concrete in contact with the reinforcement is carbonated. The maintenance of the passive film requires the presence of some oxygen, and if the supply of oxygen is very restricted. as in concrete buried under moist ground or in deep water, i t can all be used LIP.In theory, corrosion can then occur through reduction of water to H?. but in practice, corrosion rates are little greater than when a passive film exists (H63). Several approaches to the prevention of corrosion have been followed. in addition to that of making concrete that is less permeable and that gives adequate cover to the steel (T59). They include: ( I ) Cathodic protection, in which a voltage is applied between the reinforcement and a conducting paint applied to the outer surface of the concrete. (2) Sacrificial anodes, in which an electropositive metal or alloy is embedded in the concrete and connected electrically to the reinforccment. (3) Corrosion inhibitors, e.g. Ca(NO,),, which appear to act by stabilizing or otherwise altering the oxide film o r by scavenging oxygen. (4) Inert coatings of organic materials applied to the steel. (5) Galvanizing, which, however, has been considered to retard hydration and to weaken the paste-metal bond (A32.T59).
12.4 Alkali-silica reaction
12.4.1
General
The general features of alkali-silica reaction (ASR) were first described by Stanton (S119). Hydroxide ions in the pore solution react with certain types of silica that can occur in the aggregate, resulting in internal stresses that can cause expansion and cracking. Failure may occur within days or only after years. On the surface of unrestrained concrete, i t typically produces a random network of fine cracks with some large cracks ('map cracking'). In reinforced concrete, the cracks tend to form parallel to the reinforcement. Thin sections show cracks that can pass through the aggregate; a characteristic gel is present and can occur in the cracks, as rims around aggregate
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389
particles, or elsewhere in the paste (Fig. 12.3). The gel can exude from the concrete. The necessary conditions for ASR in a Portland cement concrete nre a sufficiently high content of alkali oxides in the cement, a reactive constituent in the aggregate and a supply of water. The K + and Na' are present in the cement as sulphates and in the silicate and aluminate phases. When the conipounds containing these ions react, their anions enter products of low solubility, such as ettringite, C-S-H or AFm phases, and equivalent amounts of O H - are formed. The K + or N a f are involved at this stage only in the negative sense that, since their hydroxides are soluble, they allow the O H 10 enter the pore solution. ASR is unlikely to occur in concrete made with Portland (not composite) cements if the content of equivalent Na,O (Na,O, = Na,O + 0.66K20) in the concrete is below 4 k g m - ' . and a practical limit of 3 k g m - 3 has been proposed to allow for day to day variations in cement composition (H65). An alternative criterion based on cement composition ( N a 2 0 , < 0.6%) does not allow for varying cement content in the concrete. Alkali cations may also be supplied from cxternal sources such as solutions of Na,SO, (T60,P53) o r NaCl (N21), mineral admixtures (Section 12.4.4) or aggregates. In all cases they will yield equivalent amounts of O H - except insofar as they enter solid phases or are balanced by other anions that stay in solution.
Fig. 12.3 SEM (backscattered electron image) of a concrctc showing gcl (dark patches) formed by ASR, present in the cement paste in places far removed from the reactive aggregate. Bright regions are of unhydrated clinker. Courtesy W H D Microanalysis Consultants Ltd, Ipswich, U K (H64).
390
Cement Chemistry
Diamond (D50) described the types of silica that can take part in ASR. They include quartz if sufficiently strained or microcrystalline. tridymite, cristobalite and glass or other amorphous forms, which occur in varying combinations in opals, flints, cherts and other rock types. Op,'I I 5- are especially reactive. Macroscopic, unstrained crystals of quartz appear to be unreactive but are possibly not completely inert. Some silicate minerals and volcanic glasses may undergo reactions similar to ASR. If all other variables are fixed, a curve of expansion against the percentage of the reactive constituent in the aggregate often passes through a maximum at a 'pessimum'composition. For opals and other highly reactive constituents in mortars, this is typically under 10% (Fig. 12.4) (H66) but for less reactive constituents it can be much higher and may even occur at 100%. The expansion also depends on the particle size of the reactive constituent. The results of different investigators vary considerably (H67). probably on account of differences between the materials used, but expansion appears always to be greatest for material in the 0.1-1 .O mm range and is possibly zero o r negligible if the particle size is below about 10 pm. At least some specimens of opal, if sufficiently finely ground, act as pozzolanas (BI 55).
Fig. 12.4 Relation between the expansion of a mortar (w/c = 0.41, aggregate1 cement = 2, age = 200 days) and the content of Beltane opal, expressed as a weight percentage on the total aggregate. Hobbs (H66).
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391
Fig. 12.5 Compositions of gels produced by ASR (A) in cracks or otherwise deposited within a concrete and (B) exuded to the exterior. In each case. the data are given as percentages on the ignited weight, and points to the left of the broken line merely illustrate how the results are expressed. Knudsen and Thaulow (K67).
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Cement Chemistry
12.4.2 Chemistry of alkali-silica reaction
Knudsen and Thaulow (K67) made X-ray microanalyses of the gel present in cracks in a 30-year-old concrete that had undergone ASR and compared them with typical analyses of gel exuded from the surface, reported by ldorn ( 1 15). The gel in the cracks was, in general, much higher in CaO and lower in alkalis than the exuded gel (Fig. 12.5), and its CaO content increased on moving along a crack away from its source. The authors concluded that an alkali silicate gel formed initially reacted with Ca2+ ions from the cement paste, with which the exuded material had scarcely been in contact. The compositions of the gels were analogous to those of ones in the Na20-CaOSi0,-H20 system studied by Kalousek (K19). Regourd et ul. (R69) made similar observations. Diamond et 01. (D51) observed that K i , but not Ca'. penetrated deeply into opal grains, and Bhatty (B156) found that the gel formed from C,S, opal, pfa and NaOH was essentially a calcium silicate gel and gave an XRD pattern of C-S-H(1). These results suggest that the chemistry of ASR is essentially that of pozzolanic reaction (Section 9.3.5), the different effect on concrete properties arising mainly from the difference in particle size of the siliceous material. The following explanation of the process is essentially that given by Powers and Steinour (P54) and, in more detail, by Dent Glasser and Kataoka (D52,D53). Hydroxide ions attack the silica, causing Si-0-Si bridges to be replaced by pairs of SiO- groups. This process, many times repeated, ultimately fragments the three-dimensional silica framework into separate silicate anions, which in a concentrated solution are of varying sizes. The negative charges, whether on oxygen atoms still forming part of the framework or on ones present in separate silicate ions, are balanced by the most readily available cations, which are K + and N a f . The damaged framework is sufficiently deformable that, for some compositions at least, the material can imbibe H 2 0 molecules and expand; if sufficiently fragmented, i t can be highly mobile. In these respects, it differs from C-S-H, which has a relatively rigid structure based on Ca-0 layers. It is unstable in the presence of Ca2+. which reacts with it to form C-S-H. This regenerates OH- in the solution. e.g. from CH if the latter is the source of the C a 2 + . The alkali silicate constituent of the gel can possibly itself accommodate some C a 2 + , but a point is probably soon reached at which particles of C-S-H begin to form as a separate constituent. In pozzolanic reaction, the alkali silicate gel is formed in an environment rich in C a 2 + and, except in a narrow zone close to the reacting surface, is quickly converted into C-S-H. In ASR, it is formed in an environment poor in C a 2 + ,and massive outflows of gel may result. The cement paste cannot supply C a 2 +fast enough to prevent much of this gel from persisting for long
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393
periods. This sit~iationis especially marked if the alkali silicate gel forms within the aggregate particles, as is the case with opal. The K'. Na' and O H - concentrations i n the pore solutions of mortars containing reactive aggregates are lowered due to uptake by the gel (D51). Kollek pt rrl. (K68) showed that destructive expansion was unlikely to occur i n a particular series of mortars made with pure Portland cements if the OH - concentrations in otherwise similar mortars not containing the reactive constituent had not reached 0.6 mol 1 by 180 days. This is a correlation and not a casual relationship, and i t would be wrong to regard any concentration determined in the absence of reactive aggregate as a 'trigger' value at which the aggregate begins to react. Cole et (11. (C67,C68) showed from XRD, X-ray microanalysis, IR, DTA and thermal weight loss data that the rims around sandstone and siltstone aggregates in a 30-year-old concrete contained an alkali-substituted okenite (C,S,H,) (M121) and a precursor phase characterized by a 1.22-nm XRD spacing. This latter phase was more highly hydrated than okenite, had formed in a wet environment and on being dried changed into okenite. A possible similarity to rhodesite [(Ca,K,Na),Si,,O,,~ 1 I H,O] was suggested, but the latter does not change into okenite on dehydration (G83). Okenite has a structure based on silicate sheets and layers (M121), and a higher hydrate of similar structure could well exist. Similar products have been observed by other workers (B157,D54) and had previously been tentatively identified as zeolites (C68), or jennite or tricalcium silicate hydrate (BI 57). These crystalline phases appear to be minor, though possibly equilibrium. products of ASR. A 'rapid chemical test' for the susceptibility of aggregates to ASR has been described (M122). The chemistry on which it is based is confused (D55).
12.4.3 The expansion process Expansion due to ASR is generally attributed to imbibition of pore solution by the alkali silicate gel (V10); the resulting swelling pressure is not simply related to the expansion in absence of restraint (D52). In accordance with this hypothesis, many dried sodium silicate gels swell markedly on wetting and can develop high swelling pressures (S120), whereas the equivalent volume changes for C-S-H are small. Vivian (VIO) found that reactive mortars stored for about 40 days in dry air did not expand, but if then placed in water expanded very rapidly. Expansion can also occur in a concrete that is continuously saturated; Hobbs (H65) has argued that this process is one in which a solid reactant yields a more voluminous solid
394
Cement Chemistry
product but which cannot properly be called imbibition. Osmosis h:~\been postulated (H68) but there is no substantial evidcncc that a semipermeable membrane exists. Three principal hypotheses have bccn proposed to account for the decreased expansion observed if the reactive material is more finely ground or present in quantities above the pessimum: ( 1 ) 'The gel is more thinly distributed, and C a 2 + ions are thercforc more readily available to it, accelerating its conversion into the virtually non-expansive C-S-H (P54). The thinner distribution of the gel might also in itself decrease the expansive stresses. (2) The gel has a lower ratio of alkali cations to Si02. which renders i t less expansive. Dent Glasser and Kataoka (D56) found thc expansion ol' the solid phases on placing silica gel in NnOH solutions to p ~ s s through a maximum at SiO,/Na,O ratios that might reasonably havc corresponded to pessimum compositions. Strublc and Dii~monci (S120,S121) obtained broadly similar results for swelling prcssurcs and amounts of unrestrained swelling for dried sodium silicate gels placed in water, but various anomalies led them to conclude l l i i ~ tthe swelling characteristics were probably more directly relatcd to thc anion structure of the gel. This depends not only on SiO, Nn,O ratio. but also on drying, ageing, temperature, type of silica. carbonation and possibly other factors. (3) The gel is formed before the paste has hardened sullicicntl! for damage to occur (028,H65).
These hypotheses are not mutually exclusive, though the general slonness of pozzolanic reactions renders (3) unlikely. The evidence is insullicient to determine their relative importance, which could probably be asscsscd from a combination of expansion data, pore solution analyses and S E M of polished sections with accompanying X-ray microanalyses mode in parallel on a series of mortars of suitable ages and compositions. Curves of expansion against time typically have the form shown i n Fig. 12.6. Cracking usually begins at an expansion of 0.05- 0.l%. I t is unlikely that the start of the reaction is delayed, and thus probable that a certain degree of reaction can occur without producing expansion, which presuniably begins when the gel can no longer be accommodated in the pore structure of the paste without producing stresses sufficient to disrupt it.
12.4.4 ASR in mortars or concretes made with composite cements The relations between expansion and the content and particle size of the
Concrete Chemistry
395
reactive aggregate early suggested that expansion might be decreased by the use of pozzolanic admixtures, and provided that the latter do not thcmsclvcs supply too much alkali this has been found to be the case. Hobbs (H69) reviewed and extended data on the efrects of replacements by pfa o r ggbfs on expansion in mortars and concretes. In the great majority of cases. the replacements decreased expansion, though not always sufficiently to prevent failure. In a few cases in which the admixture was itself especially high in alkalis, expansion increased. Failure sometimes o c c ~ ~ r r eatd lower concrete Na,O, contents than would have been expected if these where calculated from the contribution of the Portland cement alone. Hobbs concludcd that. contrary to some earlier views, allowance must be made for the alkalis supplied by the admixture, and suggested that, failing a direct test of the efl'ectiveness of a particular pfa or slag in preventing expansion. the Na,O, content should be calculated by adding one-sixth of the total alkali content of the pfa, or one-half of that of the slag, to the contribution from the cement.
w
Fig. 12.6 Typical relations between expansion produccd by ASR and age: data arc for mortars with wjc = 0.41 and aggrcgate/ccmcnt = 2, (A) with 6.7% of Bcltanc opal in the aggregate and a cement with Na,O, = 1.24%, and ( B ) with 4.5% of
Beltane opal in the aggregate and Na,O,
=
0.8%. After Hobbs (H66).
Kollek ct a/. (K68) extended the parallel studies on expansion and pore solution compositions mentioned in Section 12.4.2 to mortars containing pfa, slag or natural pozzolanas. For each mix, several contents of the
396
Cement Chemistrv
reactive aggregate (Beltane opal) were used to ensure that pessinium compositions were being compared. With a high alkali cement (Na,O, = 0.93%). failure occurred in the absence of an admixture, but was avoided at sufficiently high replacement levels of pfa, slag or one of the natural pozzolanas used. With a cement lower in alkali (Na,O, = 0.68%). failure did not occur in the absence of an admixture, but took place in mixes with either of the pfas, most of the natural pozzolanas and one of the slags tested. The pore solution con~positionswere consistent with these results. They could not be explained by assuming that the admixtures acted as inert diluents in regard to the alkali contents of the solutions. In the mixes with pfa, failure could be correlated with an O H - concentration greater than 0.3 mol I - ' at 180 days in an otherwise similar mix not containing the reactive aggregate. The corresponding values for mixes with slags or natural pozzolanas were 0.5 moll- ' and 0.4 mol I - ', respectively. The relations between the O H - concentration in the pore solution of a mix not containing reactive aggregate and the likelihood of a failure in a mix i n which one is present thus differ for the various types of cements. The alkali cations in pfa normally occur almost entirely in the glass. and when the latter reacts may be presumed to enter the alkali-rich silicate that appears to be the initial product. When this phase is decomposed by reaction with C a 2 + ,they will be distributed, like alkali cations from any other source. between the solution and the solid hydration products, on which they are probably adsorbed (Section 7.3.2). The C-S-H tends to take them up more strongly as its Ca/Si ratio decreases (B158,G63); consequently. the alkali cations released from the pfa are less effective in raising the O H - concentration of the pore solution than are those released from the cement. The method outlined in Section 7.5.2 for calculating the O H - concentration in the pore solution of Portland cement mix was extended to cover Portlandpfa cement mixes taking this into account (T37). 12.5 Sulphate attack
12.5.1
General
Portland cement concrete is attacked by solutions containing sulphate, such as some natural or polluted ground waters. Attack can lead to strength loss. expansion, cracking and, ultimately, disintegration. A I : I0 mortar placed in 1.8% Na2S0, solution can show a linear expansion of 0.5% within a few weeks (T60). Expansion is not always an adequate measure of strength loss. The effects are minimized in a dense concrete of low permeability and by using a sulphate-resisting cement, in which there is little or no aluminate phase.
Concrete Chemistry
397
Sulphate attack has often been discussed in terms of reaction between solid phases in the cement paste and dissolved compounds, such as Na2S0, or MgSO,, in the attacking solution. This obscures the fact that the reactions of the cations and anions in that solution arc essentially independent; for example, a solution of Na,SO, may cause both sulphatc attack and ASR (T60,P53). and one of MgSO, causes sulphate attack and reactions forming brucite. X R D shows that sulphate attack leads to formation of ettringite and often also of gypsum (H59). SEM shows cracks associated with massive growths of ettringite around aggregate particles (Fig. 12.7). Gypsum can form in similar locations. Both ettringite and gypsum can also form in pores and, in the regions that have been attacked, C H disappears and the Ca/Si ratio of the C-S-H formed in situ from the alite o r belite decreases, ultimately converting it into hydrous silica (C66,H64). The effects on the C H and C--S-H are thus similar to those produced by carbonation (Section 12.3.2). The expansion has generally been attributed mainly to the formation of ettringite and to a lesser extent to that of gypsum.
Fig. 12.7 SEM (backscattered electron image) of a concrete showing massi\e deposits of ettringite (one marked 'E'), which through sulphate attack has grown on aggregate surfaces, with associated cracking. Courtesy W H D Microanalysis Consultants Ltd, Ipswich, U K (H64).
The observed microstructures show that both ettringite and gypsum are deposited from solution, crystallization thus taking place in accordance with the following equations:
398
Cement Chemistry
The AI(0H); and O H - . and some of the Ca", needed to form ettringite could be supplied by unreacted aluminate or ferrite phase, but the main source of AI(0H); in a hardened concrete is likely to be AFm phase. represented here as monosulphate:
Whichever phase provides the AI(OH), and O H - , additional Ca" is needed to form either ettringite or gypsum. Reaction of cement pastes with CaSO, solutions is a special case, considered later. In other cases. CH dissolves and C-S-H is decalcified, in accordance with equations 12.3 and 12.4, respectively. These latter processes release a quantity of OH- equivalent to the SO:- taken up in ettringite and gypsum. I f the only cations in the attacking solution are Na' or K t , this O H - re~nairisin the solution. \vilere it augments that already present. This can cause ASR if the aggregate is reactive. In this respect, the situation is similar to that existing if the concrete is placed in a solution of alkali hydroxide, though the local concentrations of the O H could be higher if this ion is formed as a byproduct of sulphate attack. Gypsum that has been formed by the reaction shown in equation 12.8 may dissolve with precipitation of ettringite. The dissolution is represented by the reversal of the same equation, and the additional ions required could be provided by dissolution of monosulphate, represented by equation 12.9. C,S pastes are slowly attacked by 0.1 5 M Na2S04 solution; more concentrated Na,SO, solutions also attack p-C2S pastes (T60). The action is possibly due to formation of gypsum. Lea (L6) stated that NaZSO, solutions do not attack the C-S-H in cement pastes, but the SEM evidence does not support this view. While the observed decalcification of the C-S-H might be due to carbonation, which may take place more readily in material alyected by sulphate attack, it could also be a direct consequence of the latter. C - S H must be attacked in accordance with equation 12.4 if the Ca" concentration in the pore solution with which it is in contact Falls sufficiently. and the formation of ettringite might produce such a decrease. For cement pastes containing high proportions of finely ground gypsunl. Odler and Gasser (029) found a correlation between expansion and the amount of ettringite formed, but for mortars immersed in Na,SO, solutions no such relation appears to exist (T60,H59). As with Type K expansive cements, the morphology of ettringite and the time and place of its formation are probably of prime importance. The discussion on the niechanism of expansion in these cements (Section 10.2.3) is probably largely applicable also to sulphate attack, though in the latter case the ettringite is formed around aggregate particles and elsewhere in the cement paste. and not around particles of c,A,S.
Concrete Chemistry
399
12.5.2 Calcium sulphate Some aggregates, notably in the Middle East, contain gypsum. which can cause so-called internal sulphate attack. Light microscopy shows that the gypsum is replaced in sit^^ by CH, while ettringite is formed in the cement paste and can cause expansion (C69). The process entails counter migr:~tion of SO$- and OH- between paste and aggregate; within the paste, the reactions are presumably the same as in attack by SO:- from an external source. The effect is lessened by using either a sulphate-resisting or a rapidhardening Portland cement, in the latter case because the paste can become less permeable before much SO:- has been released. Despite its relatively low solubility, CaSO, present in solution can also attack concrete. In this case, the solution can provide all the ions that are needed to produce ettringite from nionosulphate, and little or no dissolution of CH nor decalcification of C--SH need occur. The saturated solution also slowly attacks C,S pastes (T60); the explanation of this is not clear. The use of abnormally large quantities of finely divided gypsum interground or blended with the cement also produces expansion (029). but little appears to be known about the microstructures that result.
12.5.3 Magnesium sulphate Some ground waters contain significant concentrations of both SO$- and MgZ+,and are more aggressive towards concrete than SO:- solutions of similar concentration in which the only cations are Na' or K'. The action of the S O ; is augmented by that of the Mg2+,which is a particular case of the more general effect of cations forming hydroxides or basic salts of Ion solubility, considered further in Section 12.6.3. The Mg2+ reacts with O H ions from the pore solution, giving brucite:
The brucite forms a hard, dense skin on the mortar or concrete, which tends to hinder further attack (L6). The O H - present in the pore solution w o ~ ~ l d soon be consumed were i t not replenished by dissolution of CH and decalcification of C-S-H, by the reactions shown in equations 12.3 and 12.4. In the reaction of a cement paste with an MgSO, solution, the CH and C-S-H thus serve as sources of both C a 2 + and OH - . According to Lea (L6), the ettringite formed by the reaction of SO:- is itself ultimately decomposed by M g 2 + , leaving a residue of gypsum and hydrous alumina. This was attributed to a decrease in the pH of the pore solution to that of saturated Mg(OH),, which is about 10.5. Such a decrease
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Cement Chemistry
could only occur if the alkali cations are lost from the pore solution. possibly by leaching. or if the SO:- concentration in the solution increases. There does not appear to be any experimental evidence with which these hypotheses may be tested. Under some circumstances, the Mg2' enters sparingly soluble phases other than brucite. Roy er crl. (R70) observed the formation of a hydrotalcite-type phase in a slag cement paste that had been treated with MgSO, solution. Its formation was possibly favoured by the enhanced availability of AI(OH), provided by the slag. Cole (C70) reported the formation of a hydrated magnesium silicate in a deteriorated concrete seawall.
12.5.4
Effects of concrete properties and cement type
The superior resistance of dense, well-compacted concrete to sulphate attack indicates that the ability of the sulphate solution to diffuse into the concrete is a major factor affecting the rate of attack. The superior resistance of calcium aluminate cement concrete appears to be due to blocking of pores by alumina gel, together with absence of CH (Section 10.1.1I ) . Autoclaved cement-silica materials are also highly resistant, probably through a cotnbination of absence of CH and AFm phases and the formation of a more crystalline form of C-S-H (B159); hydrogarnets containing even small proportions of SiO, are resistant to attack (M 123). Supersulphated cements show high resistance, probably because of the absence of CH and because much of the AI,O, in the hydration products is already present in ettringite. The mechanism whereby sulphate-resisting Portland cements provide better resistance is not known. Heller and Ben-Yair (H59) found that substitution of such a cement for an ordinary Portland cement greatly reduced expansion but did not significantly alter the amount of ettringite formed under given conditions. In contrast, Locher (L55) found that a sulphate-resisting Portland cement mortar, unlike one of an ordinary Portland cement, gave little ettringite when stored in a Na,SO, solution. Odler and Gasser (029) found that less ettringite was formed with a sulphatemisting cement, but that the difference was insufficient to account for that in expansion. In agreement with Regourd et al. (R71), they considered that the morphology of the ettringite was different; AFt phase prepared from C,A and gypsum consisted of well-formed needles, but that formed from C,AF and gypsum appeared almost amorphous in SEM. They also suggested that slower crystallization of ettringite in the iron-rich cements might allow the formation of a matrix better able to resist stresses arising from that crystallization.
Concrete Chemistry
401
12.5.5 Composite cements and sulphate attack
Partial replacement of cement by granulated blastfurnace slag often improves resistance, but the effectiveness depends on the properties and proportions of the slag. Locher (L55) found that composite cements containing at least 65% of slag were resistant to sulphate attack irrespective of the A1203content of the slag ( 1 I or 17.7%) or the potential C,A content of the cement (0-1 1 %), but that those made with lower contents of slag were resistant only if the cement was low in A1,0,. Finer grinding of the slag further improved sulphate resistance if the content of slag was over 65%, but worsened i t at lower contents of slag. With a zero-C3A cement, 20--50% replacements by the slag containing 11% of AI,O, further improved resistance, but similar replacements by that with 17.7% AI,O, worsened it. Locher concluded that resistance in slag cements arises from the type of pore structure, which at high slag contents was either less permeable or such that ettringite cannot nucleate. The efl'ects of finer grinding and of varying the Al,03 content of the slag suggest that enhanced resistance from this cause is countered by the relatively high AI,O, contents of granulated blastfurnace slags, and that to obtain it requires a proper balance between the proportion, fineness, AI,O, content and intrinsic reactivity of the slag. High sulphate resistance in a slag cement can probably only be obtained at the cost of some reduction in early strength. Other mineral additions, such as pfa, can also increase sulphate resistance (R71), but with composite cements in general, the effect is likely to depend markedly on the degree of curing before exposure to the sulphate solution begins, because of the slow reaction of the mineral addition. Hughes (H70) found that 30% replacement of cement by pfa in a paste greatly increased the time to cracking if the paste had been cured in saturated C H solution for 12 weeks before being transferred to a Na,SO, sojution, but if the preliminary curing period was restricted to 1 week. expansion took place more quickly in the pfa-cement paste.
12.5.6
Reactions involving sulphate a n d carbonate
Thaumasite [Ca,Si(OH),(S0,)(C03)~12H,0;Section 6.2.11 can form through a combination of sulphate attack and carbonation. Its crystal structure and morphology resemble those of ettringite, and its formation can cause severe softening or cracking of concrete or mortar. It can easily be misidentified as ettringite. Crammond (C32), who observed i t in failed mortars and renderings on brickwork, described the conditions conducive to
402
Cement Chemistry
its formation as a constantly high RH. temperaturc around 4 C. :~dequatc. supply of S O $ and C O i - ions. and presence of reactivc A120,. The prior formation of ettringite seems to be needed, probably as a nucleating agenc: this would explain the need for a source of AI,O,. The quantity of ettringitc. that can be formed from a cement is limited by the amount of A1,0, thaumusitc formaavailable, but, assuming a continuing source of SO: tion is limited only by the available CaO and S O 2 . I t can thurclhrc form in large quantities. reducing a mortar into a mush. Brickwork high in SO:covered with a cement-based rendering that has cracked ~hroughdrying shrinkage, and existing in a cold. damp climate, provides a typical situation Ihr thaumasite formation. Other cases in which this has been obsened include the concrete lining of a hydroelectric tunnel (BI6O). masonry in historic buildings constructed or repaired at difkrent timcs using gypsumand cement-based materials (L56) and cement bricks made with a n uggrcgate that contained a sulphide mineral (030). This last case occurred in a region (Transvaal) of relatively warm climate. Precast concrete products cured at 80•‹C have occasionally cracked and lost strength after fairly long subsequent exposure to the weather at ordinary temperature. Strohbauch and Kuzel (S122) found the following sequence of reactions to occur. During the initial curing at 80"C, monosulphatc was formed. On exposure at 22•‹C.anion exchange occurred. giving C,AC,,,,H,? and C,ACH, ,. together with gypsum, which reacted with unaltered monosulphate to give ettringite. Thaumasite was not formed. The conditions under which C O i - can replace SO:- in nionosulphate are not clear.
-.
12.6 Other forms of attack 12.6.1
Physical attack
Concrete may be damaged by frost, and especially by repented cycles of freezing and thawing. Damage usually begins with flaking at the surface. and gradually extends inwards, though deep cracks may occur. D ~ ~ m a gassocie ated with freezing does not occur unless a suflicient quantity of water is present in the pores, and is minimal in dense concrete of low wjc ratio and low permeability. It can be much reduced by air entrainment (Section 1 1.3.1 ). It is especially likely if freezing occurs before adequate hardening has occurred. Various explanations have been given and may not be mutunlly exclusi\e. Early theories associated damage directly with the volume expansion that occurs on freezing; this could cause disruption if the water that freezes is sufficiently confined, e.g. between regions already frozen. The damage could
Concrete Chemistry
403
also be caused by the growth of ice crystals, resulting from the migration of' water as a result of differences in vapour pressure. Collins (C7I) postulated a mechanism, similar to that believed to causc frost heaving in soils, in which the growth of ice lenses led to the for~nationof weak layers parallel to an exposed surface. Other theories (P55,P56.L57) attribute the damage to hydraulic pressure caused by the movement of water through capillaries. Powers (P55) considered that the expansion associated with freezing in the pores near the surface forced water inwards through small pores. \chert it could produce stresses sufficient to rupturc the surrounding material. This theory led to the successful development of air entrainment. Later work showed that a properly air-entrained concrete c~o/itrtrct.von freezing. This was attributed to migration of water out of the gel to the ice crystals. occurring because the vapour pressure of supercooled water is higher than that of ice at the same temperature (P56). Studies using low-temperature calorimetry appear to provide clear e\.idence of the formation of an ice front that nucleates at the surfxx and moves inwards (BI 12,B161). They also showed that the pattern o f ice formation is much affected by thc previous drying history. Some other forms of physical attack will be briefly mentioned. Soluble salts may be drawn into a sufficiently permeable concrete by capillary action and crystallize within the pore structure when evaporation occurs (e.g. Ref. S123). Mechanical or thermal stress or drying shrinkage can all cause concrete to crack. Thermal stresses can cause damage at low tcmperaturcs even if the pores contain little or no water. The use of de-icing salts can cause surface damage, known as salt scaling; suggested causes have included rapid cooling caused by the melting of ice, osmotic effects associated uith the presence of high concentrations of de-icing chemicals in the surface layers of the concrete and chemical attack (Section 12.6.3). Surface damage from abrasion depends markedly on w/c ratio and on the quality of the surfaco layer of the concrete; at high w/c ratios it depends also on the properties of' the aggregate. Abrasion resistance can be increased by surface treatments. e.g. with sodium silicate.
12.6.2
Leaching
Leaching of concrete by percolating or flowing water has sometimes caused severe damage, e.g. in dams, pipes o r conduits, and is potentially important for the long-term storage of nuclear wastes. Pure water may be expected to remove alkali hydroxides, dissolve CH and decompose the hydrated silicate and aluminate phases. Reference to the equilibria discussed in Chapters 5 and 6 indicates that, for practical purposes, the ultimate residue will consist
404
Cement Chemistry
essentially of hydrous forms of silica, alumina and iron oxide, all the CaO having been lost. By this stage, the cement paste will have disintegrated. The equilibria also suggest that CH will be dissolved before the othcr phases are attacked, but, in practice, attack is likely to be sinlultaneous, because of the greater specific surface areas of the hydrated silicate and aluminate phases. The rate of attack depends on the quality and shape of the concrete. the rate at which the water percolates through or flows over it. the temperature and the concentrations of solutes in the water. Attack is most likely to be serious with soft water. The partial pressure of CO, in ~mpollutedair at sea level is 32 Pa "tm). Calculation based on the data in Table 12.1 shows that, for water in equilibrium with such air and not containing any other solutes, the significant species present are CO,, HCO, and H +: [CO,] is 0.012 mmol I - and the pH is 5.6. In underground waters. [COJ may be much higher and the pH correspondingly lower. Table 12.1
Equilibrium constants in the system CaO-C0,-H20
Reaction
Definition or K
log K
[CO,~/PCO~ [H,co,l/[Co,l ( H '![HCOiI/[H,CO,I { H )[CO: -]/[HCOJ [Ca2+][CO:-]
-
-
CO, (gas) = CO, CO, + H,O = H,CO, H,C03 = H + + HCOj HCO; = H' + COiCaCO, (calcite) = C a 2 ++ C O :
+
1.42
- 2.8 -
3.5
10.25 -8.35 -
From Ref. P57. Species are in solution except where stated. Square brackets denote species concentrations: curly brackets denote activities. Concentrations are In mol l '; pcc,z isin ( 1 atni = 101 kPa).
A solution of CO, can dissolve CaCO, with formation of additional HCO J CaCO,
+ CO, + H,O
4
Ca2+ + 2HCO;
(12.1 1 )
and can similarly dissolve CH or Ca2+ and O H - ions from C-S-H or hydrated calcium aluminate phases. In all cases, the amount of CaZ' that a given volume of solution can dissolve is limited by the equilibria involving COi- (Table 12.1). The term 'aggressive CO,' is used to express this amount and the rate at which attack is likely to occur. I t is properly defined as the quantity of CO, in unit volume of solution that can react with CaCO, according to equation 12.1 1. As Rogers (R72) has noted, it has sometimes been incorrectly defined and calculated. I t is smaller than the concentration of CO, present as such or as H,C03, because when the solution dissolves CaCO, the concentrations of all the species change in accordance with the equilibria.
Concrete Chemistry
405
Rogers critically reviewed methods for determining total and aggressive CO,. Natural waters may contain other anions, such as SO:-, and in the earlier stages of leaching, at least, the situation is further complicated by the presence of alkali hydroxides from the cement pore solution. In any aqueous system of the type considered, the equilibrium composition of the solution and the nature of the solid phases can be calculated by setting up and solving equations for the various acid-base, precipitation and complexation equilibria and for charge and mass balance. The general principles of such calculations, and references to computer programs for solving the equations. may be found in works on natural water chemistry (e.g. Ref. P57). 12.6.3
Miscellaneous forms of chemical attack
This section summarizes observations on the actions on concrete made using Portland or composite cements not considered earlier. There have been few systematic or detailed chemical studies, and the information given is based largely on results reported by Lea (L6) and Kuenning (K69). Acids have an action similar in principle to that of water, but greatly intensified; CH is dissolved and the hydrated silicate and aluminate phases are decomposed, with removal of C a Z + .Those giving insoluble products. such as H,SO, or H,PO,, tend to be less aggressive for a given concentration thaa those that do not, such as HCI or HNO,. Fattuhi and Hughes (F48) made a detailed study of the action of H2S04.Acid attack is produced not only by Br~nstedacids that are neutral molecules, such as the above. but also by ones that are cations or anions such as NH;, AI(H,O);+, H2PO; or HSO,. NH; salts of strong acids, and aluminium salts of strong bases. are thus highly aggressive. Acids attack limestone aggregates as well as cement paste. Alkalis have relatively little effect, but concentrated solutions of alkali hydroxides (e.g. 5 m o l l ' ) attack concrete, probably by decomposing the hydrated aluminate phases. Metal cations that produce hydroxides or basic salts of low solubility attack cement paste. The behaviour of Mg2+ in MgSO, (Section 12.5.3) is typical, and is shown by other magnesium salts. Concentrated (3 mol I - ' ) solutions of CaCI, or MgCI, attack Portland cement concretes by reacting with the CH to form basic salts (S124). The reactions are expansive and may contribute to salt scaling (Section 12.6.1). With saturated NaCI. C,ACaCI,.10H20 is formed, and there is some loss in strength. but the effect is much less than with CaCl, or MgCI,. A composite cement containing 75% of slag was resistant to all three solutions. Some organic compounds, such as sugar solutions, attack concrete, presumably through complexing of Ca2+ and consequent dissolution of CH and the hydrated silicate and aluminate phases.
406
Cement Chemistry
12.6.4
Sea water attack
Sea water (Atlantic Ocean) contains about I I g of Na+. 20 g of C I . 7.9 g of S O : and 1.4 g of Mg2 per litre, with smaller amounts of K Ca' Hr and HCO, (0.08 g I - I ) . The pH is usually 7.8-8.3. Massazza (M 124) gave ;I general account of its action o n concrete, and Buenfeld and Newmnn (BIO.!) discussed microstructural aspects and equilibria more fully. Attack is morc serious in and somewhat above the tidal zone than on concrete that is permanently submerged. In the latter case, the combined action of Mg' and HC03 produces a surface skin, typically consisting o f a layer of brucite about 30 pm thick, overlain by a more slowly developing layer of aragonite. CaCO, is precipitated as aragonite rather than as calcite because of the presence of Mg2+ ions. This skin tends to protect a well-produced. dense concrete from further attack. To the extent that the latter occurs. the processes are those of leaching, Mg" + SO;- attack, carbonation, and CI- penetration, with the attendant possibility of corrosion of reinlhrccment, all of which are discussed elsewhere in this chapter. The efkcts of the Mg2+ and SO:- are considerably milder than those observed with MgSO, solutions of similar concentration. The danger of corrosion decreases at depth due to the lack of oxygen. In and above the tidal zone, cycles of saturation and evaporation tend to occur. the water being drawn in by capillary action or entering as spray. and daniage may occur through salt crystallization. The protective layer of brucite and aragonite is liable to be destroyed. Mechanical damage ma! result from erosion due to waves, solid debris, sand or ice. Oxygen is rendil! available, permitting corrosion to occur. As with concrete for use in other severe environments, the most important conditions for ensuring satisfactory performance are that i t should be of high cement content, of low w/c ratio, of high strength and properly made. with adequate cover for the reinforcement, and wet cured before being exposed to sea water. Mussazza (M 124) concluded that pozzolanic or slag cements ( > 60% slag) were more durable than pure Portland cements, and noted that lime-pozzolana mortars used by the Romans in sea works are still in good condition. +
'.
'.
*
12.6.5
Bacterial attack
Normal sewage is alkaline and does not significantly attack Portland cement concrete directly, but severe damage can arise through formation of H,SO, by processes in which bacterial action plays a part. Anaerobic bacteria in the sewage decompose inorganic or organic sulphur compounds. giving
Concrete Chemistry
407
H2S. This passes into the air above the liquid and dissolves in the film of water on the walls above the water level, where under the influence of aerobic bacteria, oxygen oxidizes it to H2S0,. The cement paste softens and the aggregate falls out. Preventive treatments have been of several kinds (L6). The sewage may be treated, e.g. with chlorine to oxidize the sulphides, with lime to raise the pH to above 10, which decreases the activity of the anaerobic bacteria, or with appropriate salts to precipitate the sulphide. Removal of slime and silt. in which the sulphide-forming reactions appear to occur, increase in flow and design to avoid turbulence, have been found useful. The service life of the concrete is much increased by using lin~estone as opposed to siliceous aggregates. Various surface treatments have been used, of which one of the most effective appears to be with SiF, gas. Other situations in which attack by H2S0, formed through bacterial action has been reported include cooling towers (L6) and concrete floors laid on rocks containing pyrite (FeS,) (P58).
12.6.6 Miscellaneous paste-aggregate reactions Destructive expansion occurs in concretes made with some aggregates containing dolomite, which reacts with O H - ions in the so-called dcdolomitization reaction:
The effect was first observed with certain Canadian aggregates, which also contained calcite and clay minerals. Gillott and Swenson (G84). reviewin9 theories of the expansion mechanism, concluded that the expansion was due to uptake of water by the clay minerals, which were exposed as a result of the dedolomitization reaction. In contrast, Tang ct (11. (T61) concluded from n STEM study that the clay minerals were present in the matrix material of the aggregate and not as inclusions in the dolomite, and that expansion was mainly caused by the growth of brucite crystals around the surfaces of the dolomite grains; the clay minerals provided pathways through which the alkaline solution could penetrate. Destructive reactions associated with dedolomitization have also been observed with dolomitic aggregates from Bahrein (F49), in this case possibly complicated by the presence also of gY Greening ((385) suggested that, under some conditions, ettringite c o ~ ~ be ld formed by reaction between monosulphate and calcite:
408
Cement Chemistrv
but it is not clear under what conditions, if any, this reaction takes place or can cause expansion in concrete. Van Aardt and Visser (VI I,V12) showed that felspars can react with CH and water at ordinary temperatures to give C,AH,, alkalis and other products, and that the C,AH, could react further with CaSO, to form ettringite. They concluded that some felspathic rocks, such as greywackes. and some shales and sandstones, might be suspect as regards reactivity towards cement paste. Way and Cole (W34) confirmed the occurrence of felspar reactions, but found no evidence of deleterious reactions with granite or basalt aggregates. Reinforcement of a cement paste or mortar with glass fibre much increases the tensile strength and toughness, but these properties deteriorate with time, especially in a moist environment. The deterioration is retarded. but not eliminated, by using a special glass high in ZrO,. The glass is attacked by the alkaline pore solution, but the loss of strength and toughness may not be a direct consequence of this. Changes in the microstructure at the interface between glass and cement paste take place, and may be due both to attack on the glass and to processes occurring within the cement paste (M 125,S125). Growth of products, especially CH, between the filaments of which the fibres are composed alters the mode of fracture and has been considered the dominant cause of deterioration, at least during the first year (B 163). 12.6.7
Fire damage
Because of its relatively low thermal conductivity and high specific heat. concrete provides good protection to steel against fire, though it may itself be extensively damaged. At low temperatures, cement paste expands on heating, but by 300•‹C,contraction, associated with the water loss. occurs. Normal aggregates continue to expand, and the resultant stresses lead to loss of strength, spalling and cracking. Quartz expands sharply at 573' C due to polymorphic transformation, and calcite begins to contract at 900-C due to decomposition. Blastfurnace slag, used as aggregate, is highly resistant (P59,L6). Piasta et al. (P60) described the thermal effects on the cement paste. Below 500•‹C,major effects include carbonation and coarsening of the pore structure. Decomposition of CH at 450-55O0C, and of CaCO, above 600,C. yield CaO, which rehydrates on cooling. The resulting expansion may disrupt a concrete that has withstood a fire without actual disintegration (L6). Abo\.e 500•‹C,the pore structure continues to coarsen, and incipient crystallization of P-C,S occurs. Extensive carbonation occurs in fire-damaged buildings that have been left standing for a few years (H71).
References Abbreviations used in the reference list are as follows: .?nd SCC = Symposium on the Chetnistry of' Cemcv~ts.Stocklrolnr 1938, Ingcnibrsvetenskapsakademien, Stockholm (1939). 3rd ISCC = Procecdings of' thr Third Itrterncrtioturl S~wposiunion thr Cliet~listy.(!f' Crnlent. London 1952, Cement and Concrete Association, London (1954). 4th ISCC = Chemistry of' Cement. Proceedings of' the Fourth Interncrtionrrl Sjwiposirmi. Wrrshit~gtor~ I960 (National Bureau of Standards [now National Institute of Standards and Technology] Monograph 43). 2 vols, US Department of Commerce, Washington (1962). 5th ISCC = Proceeclings 01'the F$h Int~rnrrtioncrlSjwposiut~~ on thc, Chrmistr~~ of' Ccment. Tokyo 1968, 4 vols, Cement Association of Japan, Tokyo (1969). 6th ICCC = Si.uth Internrrtiond Congress on thc Chet?ri.stq~of' Cenicwr. Moscow. 1974, Vols 1. 2(1). 2(2) and (3), Russian with English preprints. Stroyizdat. Moscow (1976). 7th ICCC = 7/11 Intc~rnutioncrlCongress on the Chemistrj~of' Cement, Paris, 1980. 4 vols, Editions Septima, Paris (1980, 1981). 8th ICCC = 8th International Congress on the Chemistrj, (?/'Cement,Rio de Janeiro. 1986, 6 vols, Abla GrLfica e Editora, Rio de Janeiro (1986). ICCem. Microsc. = Proceedings of' the International Conf2renc.e on Cement MicroInternational Cement Microscopy Association, Duncanville. TX, USA. .c.c~)p.~,,
Aggarwal, P. S., Gard, J. A., Glasser, F. P. and Biggar, G . M. (1972). Con. Concr. Res. 2, 29 1 . Amafuji, M. and Tsumagari, A,, in 5th ISCC, Vol. I, p. 136 (1969). Allen, T. Particle Size Measurement (2nd Ed.), 454 pp., Chapman and Hall. London ( 1975). Anderegg, F. 0. and Hubbell. D. S. (1929). Proc. Am. Soc. Testing Muter. 29. 554. Anderegg, F. 0. and Hubbell, D. S. (1930). Proc. Am. Soc. Testing Mutc~r.30. 572. Aldridge, L. P. (1982). Cem. Concr. Res. 12, 381. Ahmed, S. J. and Taylor, H. F. W. (1967). Nature 215, 622. Allmann, R. (1968). Neues Jahrb. Mineral. Monatsh. 140. Ahmed, S. J., Dent Glasser, L. S. and Taylor, H. F. W., in 5th ISCC, Vol. 2. p. 118 (1969). Aruja, E. (1960). Acta Cryst. 14, 1213. Allmann, R. (1970). Chimicr 24, 99. Allmann, R. and Lohse, H.-H. (1966). Neues Jahrb. Mineral. M O I I N I S161. ~. Abriel, W. (1983). Acta Crj~st.C39, 956. Afshar, A. B. and McCarter, W. J., in 8th ICCC, Vol. 3, p. 193 (1986). Alford, N. McN. and Rahman, A. A. (1981). J. Muter. Sci. 16. 3105. Allen, A. J., Windsor, C. G., Rainey, V., Pearson, D., Double. D. D. and Alford, N. McN. (1982). J. Phys. D15, 1817. Alexander, K . M., Taplin, J. H. and Wardlaw, J., in 5th ISCC, Vol. 3. p. 152 (1969). Aldridge, L. P., in 7th ICCC, Vol. 3, p. VI-83 (1980).
Cement Chemistry
Alford, N. McN.. Birchall. J . D., Howard. A. J . and Kendall. K. (1985). .I. :Mrr/c,r.Sci. 20, 1 134. Abo-El-Enein, S. A,, Daimon, M., Ohsawa, S. and Kondo. R. (1974). ('cur. ('ot~(,r. Rcs. 4 . 299. Aitcin. P. C.. Carlcs-Gibcrgucs. A,, O~ldjit.M. N . and Vaquicr. A , . in 8/11 ICCC. Vol. 4, p. 22 ( 1986). Aitcin. P.-C.. Sarkar. S. L. and Diatta. Y. (1987). Mrr/c~r.Rc,.c. Sot,. Sj.tnp. PIYK.85, 26 1 . AfShar, A. B. and McCarter, W. J. (1 985). .I. Moicr. Si,i. L-crr. 4 . 85 I . Agarwal. R. K.. Paralkar, S. V. and Chatterjcc, A. K., in 8/11 IC'('('. Vol. 2. p. 327 ( 1986). Asaga. K. and Roy, D. M. (1980). C'cwr. ('otlc,r. Ros. 10, 287. Andersen. P. J.. Roy, D. M. and Gaidis, J . M. (1988). C ' c w . C'otrc,r. Rcs. 18, 980. Arlig~lic,G.. Ollivicr. J. P. and Grandct. J. (1982). C ' c w . C'otrcr. Rw. 12. 79. Alunno-Rossetti, V., Chiocchio, G. and Collcpardi. M. (1974). Ccw. C'ot1c.r. Rcs. 4. 279. Aitkcn. A. and Taylor, H . F. W., in 4rh ISCC'. Vol. I , p. 285 (1962). Alexanderson. J. (1979). Ccw. Cot1c.r. Res. 9, 507. Al-Khalaf, N. N. and Page, C. L. (1979). Ccw. C'otlcr. Rcs. 9. 197. Arliguic. G.. Grandct, J. and Duval, R., in 7/11 IC'CC', Vol 3. p. VII--22 ( 1980). Andrade, C., Alonso, C.. Santos, P. and Macias, A,. in 8/11 ICC'C'. Vol. 5. p. 256 (1986). Bigare, M., Guinier, A., Maziercs, C.. Rcgourd, M., Yannquis. N.. Eysel. W., Hahn, T. and Woermann, E. (1967). J. At~r.Cc,rittir. Sot,. 50. 609. Boikova, A. I.. Esayan, A. and Laz~lkin,V., in 7/11I('<'('. Vol. 4. p. I83 (1981). Boikova. A. I.. Fomicheva, 0. I . and Zubckhin, A. P., in 8/11IC'('('. Vol. 2. p. 243 (1986). Boikova, A. I., in 8/11ICCC. Vol. 1 . p. 19 (1986). Burke, E.. in 3rd ISCC. p. 50 ( 1954). Barnes, P.. Fentiman, C. H. and Jef'f'cry, J. W. (1977). Actlr Cr.j..ui.A36. 353. Brcdig. M. A. (1950). J. Am. Cclrrrttl. Sot,. 33, 188. Barbicr. J. and Hyde. B. G. (1985). Acto Cr~vsr.B41. 383. Biggar. G . M. ( 197 1 ). Crnl. Cotrc~.Rex. 1. 493. Banda, R . M. H. and Glasscr. F. P. (1978). ('m.C'otrc.r. Rc.s. 8. 665. Berggrcn. J. ( 1971 ). Actrr C l l r t ~Sc.ctnil. . 25, 36 16. Biissem, W., in 211dSCC. p. 141 (1939). Bertaut, E. F., Blum, P. and Sagnicres, A. (1959). Ac/u Cy..c.t. 12. 149. Bachiorrini, A. (1985). Crm. Corlcr. Rcs. 15, 167. Boyko, E. R. and Wisnyi, L. G. (1958). Acrcr Cr!s.c./.11, 444. Biissem. W. and Eitel. W. (1936). Zcit. Krist. 95, 175. Bartl, H. and Schellcr, T. (1970). N e ~ mJitlirh. Miticrirl. Mot~cttsl~. 547. Brisi. C. and Borlera, M. L. (1983). Ccmcwto 80. 3. Borlera, M. L. and Brisi, C. (1984). Cc~nrctzto81, 13. Brisi, C., Borlcra, M. L., Montanaro, L. and Negro, A. (1986). Coir. Cotl~,r. Res. 16, 156. Brothcrton. P.D.. Epstein, J. M., Prycc, M. W. and White. A. H . (1974). A~rs/ritl.J. CIICWI. 27. 657.
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X3 YI Y2 Y3 Y4 Y5 Y6 Y7 YX
Z1 22 23 24 z5 26 27 28 z9 ZIO Zl l 212
Ynmnguchi. G . and Takagi, S., in 5/11 ISCC. Vol. I , p. 181 (1969). Yannaquis, N. and Guinicr, A. (1959). Bltll. Sot. Frtrn(.. MinCrtrl. ('I.I,.S/. 82. 126. Yamanaka, T. and Mori, H. (1981). Act(/ Cryst. B37, 1010. Yoshioka, T. (1970). B~tll.Cl1o11.Soc. J ~ I ~43, N I 198 I 1 , 23 17. Young. J. F. and Hanscn, W. ( 1987). MNICI.. Rc.s. Soc. Sjwlp. Pror.. 85, 3 13. Young, J. F. (1988). J. Anr. C ( , I . N ~Soc. I . 71. C1 18. Yan, R.-Zh., Ouyang, 91.-X. and Gao. Q.-Y. (1983). Silica/cs 111tl.12, 3. Young, J . F. (1972). C U H .Concr. Rcs. 2, 41 5. Zheng. Y. and Yang, N.. in Xlh ICCC, Vol. 2, p. 293 (1986). Zhmoidin. G . I . and Chatterjee, A. K . (1984). C(WI.Concr. Rcs. 14. 386. Zhmoidin, G . I . and Chatterjee, A. K. (1985). Cem. C o n c ~ Rr.s. . 15. 442. Zhmoidin, G . I . and Chatterjee, A . K. (1974). Ino,:q. Mowr. 10. 1584 (translated from Ncor;q. Mrrro. 10, 1846). Zur Strasscn, H. (1957). Zcnl.-Kdk-Gips 10, I . Zicglcr, E. (197 1 ). Zcw.-Kdk-Gips 24, 543. Zur Strassen, H. (1959). Zcnlcwt Bcron (16) 32. Ziirz. A. and Odler, I . (1987). Adv. Cerm Res. 1, 27. Zur Strasscn. H. and Stratling, W. (1940). Zcit. A n o y . AlLq. C11cw1.245, 267. Zur Strassen. H. (1958). Zcn~.-Kdk-Cips11. 137. Zclwcr, A,, in 7/11 ICCC, Vol. 2. p. 11-147 (1980). Zakharov, L. A , , in 6th ICCC, Vol. 3 , p. 153 (1976).
Appendix
Calculated X-ray powder diffraction patterns for tricalcium silicate and clinker phases
The patterns (T5) were calculated as described by Morris rt dl. ( M 16). The columns headed d(nm), I,,,, 17, k and I give the integrated intensity pattern as defined by these authors, i.e. they refer to individual reflections, which are liable to be components of overlaps. I,,, is the integrated intensity relative to a value of 100 for the strongest. Reflections with lint < 1.5 are not listed. The column headed I,,, gives an indication of relative peak heights assuming typical experimental conditions using a diffractometer, but these depend on resolution and on any variation in cell parameters. because these affect separations between overlapping peaks. They take into account contributions from reflections too weak to appear in the list of integrated intensities, and their spacings sometimes differ significantly from those of the individual components. Peaks of significant intensity can arise from groups of reflections all of which are too weak to be listed individually; the more prominent are noted in footnotes to the tables. Except with T , C,S, the cell parameters and compositions assumed are typical ones for phases in Portland cement clinkers. No data are available from which i t would be possible to calculate a pattern for an M , alite. Reference intensity ratios. I,,,/I,, (H23.M 16) are also given; their use is explained in Chapter 4.
448
Cement Chemistry
Table A.1
Calculated X R D powder pattern for T I tricalcium silicate*
"Pattern calculated from the data of Golovastikov el ul. (G2); u = 1.167nm, h = 1.424 nm, = 1.372 nm, a = 105.50", P = 94.33". y = 90.00". Other peaks of significant helght: 0.3717nm(3), 3 1 - 1 +;0.2099nm(4), - 1 4 4 +. I,,,/I, =0.798. t.
Appendix
'Table A.2
d(nm)
449
Calculated X R D p o w d e r p a t t e r n f o r M, alite*
I,,,
I,,
/I
k
I
* Pattern calculated assuming u = 3.312 nm, h = 0.7054 nm, c = 1.855 nm. 0 = 94.17 ; atomic parameters given by Nishi e / ul. (NI); site occupancies (Ca,,,,Mgo,,,Alo,,,,,Fe0,003)3 (Sio,,,AIo,03)0,. Other peaks of significant height: 0.3248 nm (2), -2 2 2 +; 0.321 1 nm ( I ) . 10 0 I + ; 0.2673nm (3). -8 2 1; 0.2562nm (2). - 8 0 6 f ; 0.250411111 (I). 2 2 5 + : 0.2276nm(2), - 1 3 4 +;0.2097nm(2). - 1 3 4 + ; 0 . 2 0 3 6 n m ( I ) . 16 0 I+;0.1861 n m ( 1 ) . -16 0 5 + ; 0.1794nm (2), -8 2 8 + : 0.1714nm (I), -7 1 l o + ; 0.1693 nm (2). - I8 0 5: 0.1691nm (2), 14 2 5 + ; 0.1594nm (I). -13 3 5 + ; 0.1561nm (1). -18 0 7. 1 11, = 0.742.
_,,,
450
Cement Chem~stry
Table A.3
Calculated X R D powder pattern for a belitc*
* Pattern calculated assuming u = 0.550 nm, h = 0.675 nm, c = 0.933 nm, P = 94.40 : atomic parameters given by Jost P I 01. ( 5 2 ) ; site occupancies ( K Na Ca,),,,M , g ,,,,,, ),(FC,,,,~ Al,n6Sio,90P S O o l ) O Other peaks of significant height: 0.1760nm (3). -3 1 I: 0.1656nm (2). - 2 3 2 +. I /I, = 0.724.
,,
,,,,.
,,,,
_,,,
,,,,
451
Appendix
Table A.4
Calculatcd X R D powder pattern Ihr a cubic aluminate phase* ---
tl(nm)
I,,,
0.6815 0.5080 0.4595 0.4227
2 3 2 5
0'4073 0.4073 0.38 10 0.3326 0.3048 0.2830 0.2782
7 3 3 2 4
0.2782
2
-
I,, 2 4 2 6
I
l4 3
3 2 5
-
11
X.
I
I i I 0 1
0 2 I 2 2
2 2 3 3 3
{2
1
3 4 4 4 5
0 2 3 0
0 1 0 2
'b{: 1
* Pattern calculated assumlng c1
:
cI(nn1) 0.2694 0.3410 0.2380 0.2200 0.3037 0.1951 0 . 1905 0 8 0 0.1822 0.1555
I,,,
I,,
11
X
I
100 2 2 7 2 3 31 2 2 34
100 3 3 7 2 3 29 4 3 30
0
3 3 4 3 3
4 6 5 4 6 6 8 8 6 8
O 0 4 2 3 -
O
1
1
0 0
3 4
3
5
= 1.5240 nni, space group Pa3, atomlc parameters given by Mondal and Jeffery (MI 1 ) and composition ( K , Na,, ,,,Ca,,,Mg,, ,,,Ti,, ,,,)(Fe,,& I , ,, Si,, ,,)Oh. Other peaks of significant height: 0.6215 nm (2). 1 I 2; 0 2272 nm (2). 4 2 5 + : 0.2177nni (2). 2 3 6 + ; 0.19X4nm (2). 3 1 7+: 0.1643nm (2). 5 5 6 + . 1,,,,/1, = 3.06.
452
Cement Chemistry
Table A S tl(nm)
Calculated X R D p o w d c r p a t t e r n for a n o r t h o r h o m b i c :ilurninatc phase*
lint I,,
11
k
1
d(nm)
lint I,,
h
k
I
* Pattern calculated assuming cr = 1 .O879 nm, h = 1.0845 nm, c = I .5 I06 nm, space group atomic parameters and site Pbca, composition (Na,,2,2Ca2,7,2)(Feo,150A11,725Sio,125)06, occupancies given by Nishi and Takeuchi (N4). Other peaks of significant height: 0.5414 nm (2). 0 2 0; 0.5388 nm (2), 1 1 2; 0.2639 nm (2). 0 2 5; 0.2586 nm (2). 3 2 3 + ; 0.2425 nm (3). 3 3 2; 0.2047 nm (2). 1 5 2; 0.2042nm (2). 2 4 4; 0.1827 nm (2). 1 3 7 + ; 0.1616nm (2). 6 1 4. 12,, I, = 1.94.
453
Appendix
Table A.6 Calculated X R D p o w d e r p a t t e r n f o r a clinker ferrite phase* ti(nrn)
fin,
I
h
* Pattern calculated assuming
k
I
d(nm)
I,,,
I,,
It
k
I
u = 0 . 5 5 3 5 nm, h = 1.4642 nm, (, = 0 . 5 3 2 8 nm. space group Ibm2, atomic parameters given for C 4 A F by Colville and Geller ( C 3 ) and composition Ca2[AI,,,Fe, ,](AI,,Fe,,,)O,, where square and round brackets denote octahedral and tetrahedral sites, respectively. I , , , / I , = 1.79.
Index Substances arc indexed under the most commonly used designations; thus, the m21.jor phases of Portland cemcnt clinker, natural minerals and common chemical compounds are usually indexed by name, and other compounds of specific importance lo cement chemistry by cement chemical formulae. Broad categories of compounds (e.g. calcium magnesium silicates) are indexed by name.
Abbreviations. 4-5 Abrasion of concrete, 328, 403 Accelerators calcium aluminate cements. 330-33 1 Portland cemcnt. 357-362 Acceleratory period calcium silicate pastes. 160. 164-165 Portland cement pastes, 223-224. 226-227 Acetone, reaction with C,S pastes, 125 Acid attack calcium aluminate cements, 333 Portland o r composik cements. 405 Additions. mineral. definition, 276 Additives, definition. 345 Admixtures (see also under specific 1) pcs), 345-362 definition, 345 for calcium aluminate cements. 33033 1 for oilwcll cements. 372-373 Adsorption loop and train modes. 355 of retarders. 345-349 of w t e r reducers and superplasticizers, 352. 354-357 Aerated concretes, autoclaved, 366 AFm phases (see also formulae of individual compounds) alkali substitution, 173 as a hydration product of pure compounds, 193-197
AFm phases (tour.) as a normal hydration product of Portland cement, 199-202, 204, 206, 208-209. 2 1 1-2 12. 2 14-22>, 225, 226. 232. 233 content o f F c , 211, 212. 214. 215 content of SO,, 200-202, 209. 21 I . 2 16 crystallinity, closeness of mixing with C-S-H. 199-200, 202, 204. 209, 211, 214, 215, 221 morphological o r microstructural aspects, 222, 225 quantity formed, 209. 217-221 relation to heat evolution, 226. 232 relation to thcrmogravimetric curve, 208, 220, 221 X-ray, DTA and infrared evidence. 199-202 as a product resulting from flash set. 233 crystal, optical, thermal and other data, 169-1 77 crystal structures, compositions. 167I69 AFt phases (see also ettringite, thaumasite and formulae of other AFt phases) as a hydration product of Portland cement, 199-202, 208, 2 16-226, 228, 232, 233 content of SO,, 180
456
Index
AFt phascs ( c o t u . ) morphological or niicrostructural aspects, 216, 221-225, 400 quantity formed, 208, 2 17-221, 228. 400 relation to heat evolution, 226, 232 relation to thermogravimetric curve, 220 X-ray, DTA and infrared evidence, 199-202 as a hydration product of pure compounds, 194-197 crystal. optical. thermal and other data, 178-181 crystal structures, compositions. 177I80 Afwillite, 142 Aggregate-cement paste interface, 37738 1 calcium aluminate cements, 328 composite cements, 38 1 Air entraining agents, 351, 403 Akernianite, 38, 49 Alcohols, polyhydric, as grinding aids, 352 Alinite and alinite cements, 343-344 Alite (see also tricalcium silicate), 5-15 chromium substitution, defects, 121 crystal structure and data for polymorphs, 5-8 density, 15 factors affecting crystal size, 82-83,105 factors affecting reactivity, 1 19-122 fluoride substitution, 57-58 formation in cement kiln. 60-61. 8 1-83 hydration, see calcium silicate pastes, Portland cement pastes in clinkers compositions. 8-1 1 effect of cooling rate, 88-89 polymorphic types, 9, 12-14 zoning, 12 natural occurrence, 15 optical and thermal data, 15 polymorphic transitions in cement kiln, 12-13, 83 polymorphism. 5-9, 12-1 3 re~r~staliization and crystal growth, 13, 82-83
Alitc ( c o t l r . ) solid solutions o r ionic substitutions. 8-13 X-ray powder patterns, l3--l4, 449 zinc substitution, dcfccts, 121-122 Alkali and alkali calcium sulphates data, equilibria. 52-56 efkcts on cement hydration. 236-237 in Portland ccmcnt clinkcr, 89-91. 103, 114-115. 117 Alkali attack calcium aluminate cements. 333-334 Portland or composite cements, 405 Alkali carbonates effect on cement hydration, 361-362 in cement clinkers, 54 Alkali silica reaction, 388-396 similarity to pozzolanic reaction, 392-393 sources of hydroxyl ions, 389, 396, 397 Alkaline hydrolysis, of calcium aluminate cements, 334 Alkalis, effects of on CaO-A120,.-H,O system, 33 1 on CaO-Si0,-H20 system, 158 on Portland cement hydration, 236237 Alkalis in pore solutions composite cement pastcs, 395-396 Portland cement pastes, 227-230 Alkalis in Portland cement making, 74, 76, 78, 89, 92, 93 Alkyl aryl sulphonic acid salts, 351 Alumina, hydrous, from calcium aluminate cements, 320-325 Alumina ratio (alumina modulus), 61, 80, 85-87, 103 Aluminate (phase), 23-28 alkali substitution. 24-25 crystal structures and data for variants, 23-26 density, 28 enthalpy change on formation, 64 enthalpy change on hydration, 197I98 equilibria, 35, 39, 42, 45, 48 factors affecting reactivity, 1 19-1 22 formation in cement making, 60-61, 73-74, 84-88
Aluminatc (phasc) ( ( . o n [ . ) hydration in Portland cement pastes, 205206, 22 1-228, 23 1-237 of pure compound, 193-1 94 of pure compound in presence of sulphate, 194-196 in clinkers, compositions and modifications, 10-1 1. 2 6 2 8 ionic substitutions, 24-25, 42 optical properties, 28 structural variants, 24-26 X-ray powder patterns, 26-27, 451452 Aluminium hydroxide formation from calcium aluminate cements, 320-325 solubility, 189-190, 324 structure, polymorphism, 184-1 85 Aluminium salts, attack on concrete, 405 "Aluminosulphate", see C,A$ Aluminous cements, see calcium aluminate cements Amines, as grinding aids, 352 Ammonium salts, attack on concrete, 405 Analytical electron microscopy general principles, 132-1 33 of calcium silicate pastes, 132-1 33 of pfa cement pastes, 296297 of Portland cement pastes, 2 1 1-2 12 of rice husk ash-C,S pastes, 31 1 of slag cement pastes, 284 Andreason pipette, 97 Anhydrite data, equilibria, 52-56 effect of admixtures of hydration. 359 effect on cement hydration, 234 in cement making or clinker, 74, 83, 89-9 1 solubility, 188 'soluble', see calcium sulphate phases Aphthitalite data, equilibria, 52-56 effect on cement hydration, 237 Aqueous solution equilibria, calculation, 323-324, 404-405 Aragonite, from sea water attack, 406
Arcanite, data, equilibria, 52-56 Ash deposition, and light microscopy, lo4 Autoclaved materials, 365-371 resistance to sulphate attack, 400 very high strength. 375 Autogenous shrinkage, 270 Backscattered electron imaging (see also scanning electron microscopy) general principles, 105-1 06 Bacterial attack on concrete, 40-07 Belite (see also dicalcium silicate polymorphs), 15-23 effect of pfa or pozzolanas on hydration, 293, 305 formation in cement making, 60-61. 74, 75, 82, 86, 88 hydration, see calcium silicate pastes, Portland cement pastes in clinkers cell parameters, 22 compositions, 10-1 1, 21-22 lamellar textures, 19-20 optical properties, 22 polymorphic types, 20 X-ray powder patterns, 20, 450 . in low energy cements, 341-343 modulated structures, 21 natural occurrence, 23 reactivities of different polymorphs, 1, 19, 122 Bentonite, in oilwell cementing, 373 BET method cement pastes, 259-260 unhydrated cements, 98-99 Bingham model, 244 Blaine method, 98 Blastfurnace slag (see also slag cements; supersulphated cements) and alkali silica reaction, 395-396 and sulphate attack, 401 formation, treatment, composition, properties, 277-282 mixtures with calcium aluminate cements, 332 use in composite cements, 278-290 Bleeding, 246 Blended cement, definition, 276
458
Index
Bogirc calculat~on,62-63. 1 i4 sources of error, 1 13, 1 18 Bond, cement-aggregate, scc aggrcgatccement paste inlcrliicc Boratcs. and Portland cement hydration, 361 Bottle hydration, 147 Bound water (chemically bound water) calcium silicate pastcs. 130-1 3 1 composite cement pastes, 289-30 1 definition. 130 Portland cement pastes. 206, 219 effect of curing temperature, 364 Bredigite, 23, 49 Brownniillcrite (see also ferrite (phase)). 79
-7 L
Brucite crystal structure. data, 184 formation by reactions of dolon~itic aggregates, 407 formation on hydration of Portland cement. 216 in concrete attacked by ground o r sea waters, 399, 406 Brunauer model (paste structurc), 252 Brunauer-Emmett-Teller method, see BET method Burnability. of cement raw mixes, 7981 Burning conditions of clinker, and light microsco~v. . . 104 Butyl phosphate, and air entrainment, 35 1 CA data, equilibria. structurc, polymorphism, 35 in calcium aluminate ccmcnts, hydration, 3 17-334 superficially hydroxylatcd, 323 CAHI" crystal data, thermal behaviour. 183I84 formation from calcium aluminate cements; equilibria, 320-326 CA, data, equilibria, structure, 35 in calcium aluminate cements, hydration, 31 8-319
CA,,. 36. 318 C2A, 28 C2AH, phascs (x = 5. 7.5 or 8: scc ;iIw AFm phases) data, structures. thcrmal bcli;i\ lour-. 171, 172. 1 7 4 1 7 7 equilibria, 189, 320 391. 323 324 formation from aluminate pll;~sc..194 formation from calcium aluminatc ccmcnts. 320 et sccl. preparation. 192 C,ASH, (strltlingite; see also AFm phases) data, structure. thcrmal bcha\inur. 171 -177 equilibria. 190- 19 1 formation from calcium al~rminatc. ccmcnts, 328. 332 formation from composite cc~iicnts. 304, 3 1 1 prcparation. 304 C,A and substituted nioditications. aluminate (phase) C,AH, (see also hydrogarnet pliascs) data, structurc. solid solutions. I8 I equilibria, 189-190, 320-331. 323 324 formation from aluminate phaw. 194 formation from calciun~duminatc. cements, 320, 326-328 prcparation. 193 C,ACaCI,.H, (see also AFm phase\). 171-172 C,ACaCI;H (Friedel's salt: s c . ~;11w AFni phases) data, structure. solid solutions. thermal behaviour. 171- 177 in cement pastcs, 386. 405 C3A.3CaCI,.H,,, (sec also AFt p1i:tsc.s). I80 C3A.3CaSO,.H,,, see AFt phases: ettringite C,A,M, 49-51 C,AiCaF,, 58 C,AC,,,H, phases (x = 6.5-12: see also AFm phases) data, structures, thermal beha\ iour. 1 69-1 76 formation from Portland cement. 200
,
Index
C,ACH, phases (x = 6-1 1; see also AFm phases) data, structures, thermal bchaviour. 169-177 cquilibria, 324, 332 formation from calcium aluminate cements, 328, 332 for~nationfrom Portland ccmcnt, 200, 3 12. 38 1 C,AF, see ferrite (phase) C,(A,F)H, phascs (see also AFm phases). 175 C,AH, phases (x = 7--19; see also AFni phases) data, structures, thermal bchaviour. 169-177 equilibria. 189-190 formation from aluminate or fcrritc phase, 194-1 96 formation from Portland ccmcnt, see under AFm phases preparation, 192 C,ASH, phases (see also AFm phases). 174 C,ASH, phases (x = 7-16; nionosulphatc; see also AFm phascs) data, structures. 169-1 77 equilibria. 190 formation from aluminatc or fcrritc phase. 195-196 formation from Portland cement, see under AFm phascs preparation. 193 solid solutions. 173, 175 C,A,. 38 C,A,H, 38. 177 C,A,S ("alu~ninosulphatc") data, structurc, preparation. equilibria. 52-56 in cement making, 74-76 in expansive cements, 336-339 in low energy cements, 341-342 in rapidly setting and hardcning cements. 339-34 1 Cs43 conditions of formation, structure, data. 37. 38 C,AC,H,, (see also AFt phases), 180 C,AH,, (see also AFt phases). 180 C,AS,H,, (see also AFt phases), 180
459
C , A S , H , ? . see AFt phascs: cttrinytc formation from Portland ccmcnt. see under At-( phascs C,,AS,H,,, (see also Al-t phascs). IS0 C7A,M, 49-51 C , , A 7 C a C 1 2 ,30. 343 C,,,A7CaF2 In rapidly setting and hardcning ccmcnts. 339 340 structurc. data. 36 37 C , ,A;CaS. 37 C12A7 absence in Portland cement clinkcrh. SO d.'ltd.. b. t r i ~ c t ~ ~thcrmi~l re. bch;~viour. 36--37 derived s t r ~ ~ c t ~cont;~ining ~res additional anions. 36 37 ni ccmcnts. in c a l c i ~ ~ aluminak hydration. 318. 319. 322 3 3 in ccmcnt making. 61. 74 75 C 1 2 A 7 H .36. 176 CF. 41 42 C F 2 , 41 42 C,F (scc also k r r ~ t c(phase)). 28. 11. 42. 49 C,FH,, (scc also Iiydrog;~rnctp l i ; ~ z c ~ ) . 181-183 C,F.C;1Cl2.H (scc also AFm phazcs). 175 C,FSH, ph:~scs (see also AFm phazcs). 175 CS. structures. polymorphism. data. 3.3 C,S. scc bclitc: dicalcium sil~catcpol!morphs C2S pastcs, scc calcium silicate pastes (I-C2SH (tx-dicalcium s~lic:~tc hydrate). 364, 367-369 C,S, see alitc; tricalcium silicate C , S pastcs, scc calcium silicate paslcs C,S2, structures. polymorphism. data. 33 C,SSCH see AFt phases: thau~nasitc C,S2,CaF2, s t r u c t ~ ~ r stability. c. 5657 C,S,S ("silicosulpharc") data, structurc. preparation. equilibria, 53 56 in cement making. 74 76 C,,S,S,H (hydroxyl-ellcstadite). 74 C,,,S4CaF2, structurc, stability. compositional variation. 56 57
,,,
,,.
460
Index
C2,Al,M,S, ("Phasc Q"). 49. 50 C--S-H (C-S-H gel) at paste-aggregate intcrliicc. 377- 379 carbonation, 124. 384-386 decomposition on sulphatc attack. 397-399 definition. 123 formation in autoclave, 367 mechanism of growth. 348 3 4 9 preparations with additional ions. 158. 213-215 solubility curves, 153- 155 structural models. 142- 152 thcrmochcmistry. thermodynamics. 157.- 158 tobcrmoritc and jennite type structures in. 150-1 52 C-S-H in calcium silicate pastes Ca/Si ratio, 128- 130. 132- 133 density. 140- 141 infra red spectrum. 142 kinetics and mcch:inism of li~rmation, 159-1 66 morphology. microstructurc, 133 137 silicate anion structure. 137-140 thcrmogravinictry and DTA, 129 water content, 130 13 1 X-ray powder pattern, 140-1 41 C-S-H in composite cemcnt pastes microsilica cements, 306, 308 pfa cements, 296-30 1 pozzolanic cements. 305 rice husk ash cements, 31 1 slag cements, 284-289 supersulphated cements, 290 C-S-H in Portland cement pastes calorimetry. energetics of formation, 226, 230-232 composition, 209-2 18, 220 kinetics of formation, 228, 233, 241 microstructure, 202-204, 22 1-225 quantity formed, 218-219 silicate anion structure, 212-213 thermogravimctry, 207-208, 220-22 1 X-ray pattern, DTA and infrared evidence, 199-202 C-S-H(di, poly), 146 C-S-H(1) equilibria, 153-1 56
C--S--H(l)( c o t 1 1 . ) preparation. thermal properties. X-ray pattern. 142-143. 145-148 relation to 1.4 nm tobcrmoritc. silicate anion structure. 146--152 C-S - H ( I I ) . 142. 148. 155 Calcite. see calcium carbonate Cill~i~1111 aluminate cements. 316-334 bchaviour with aggressive agents. 316. 333 -334 hydration chemistry. 3 19-325 rnnn~~flicture. phosc composition. 317-319 setting, hardening, conversion. 325 330 use in M D F cements. 375-376 use in mixtures with Portland cement, etc.. 330-333 use in refractory concrete. 334 Calcium aluminatc hydrates (see also formulae of individual con~pounds) preparative methods. 192-193 Calcium carbonate as addition to Portland cements. 312 reaction with calcium aluminatc ccnient paste. 328. 332 reaction with Portland cement paste. 312, 381 thermal decon~position.64-65. 71-71 Calcium chloride and corrosion of reinforcement. 383. 387-388 as accelerator for Portland cemcnt. 357-361 attack of concentrated solutions on concrete, 405 in manuhcture of alinitc cements. 343-344 Calcium compounds (see also lbrmulae or names of individual compounds) attack by atmospheric carbon dioxide, 124, 168, 200, 383-386 attack by carbon dioxide solutions. 403--405 Calcium hydroxide (portlandite) at paste-aggregate interface, 377-379 data, structure, solubility, thermal behaviour, 125-1 28
Index
Calcium hydroxidc (c,otit.) determination in calcium silicatc pastes, 128-1 30 in Portland cement pastcs, 207-208 formation from calcium silicates. 128, 132133, 136-137. 140-141. 162-166 from microsilica cements, 306-308 from pfa cements, 293-294 from Portland cements DTA, infrared and X-ray evidence, 199-202 microstructure, 202-203. 209, 21 1, 214. 223. 242 quantity formed, thermogravimetry, 207-208, 2 17-22 1 kinetics of formation, 226, 233 from pozzolanic cements. 304 from rice husk ash cements, 31 1 from slag cements, 282-289 Calcium langbeinite effect on cement hydration, 235 structure. data. equilibria. 52-56 Calcium magnesium aluminates, 4 8 4 9 Calcium magnesium silicates, 4 8 4 9 Calcium oxide (see also free lime), structure, data, 33 Calcium potassium sulphate. see calcium langbeinite hydrated. see syngenite Calcium silicate bricks, 366 Calcium silicate carbonates (see also spurrite), 58 Calcium silicate chlorides, 344 Calcium silicate fluorides, 56-58 Calcium silicate hydrates, formed hydrothermally, 367-37 1. 374 Calcium silicate pastes (see also C-S-H), 123-1 66 action of accelerators, 358-362 calorimetric curves, 160, 166 early hydration reactions and products, 162-1 66 hydration kinetics and mechanisms, 159-166 microstructures. 133-1 37 phase compositions, 123, 128-1 30 thermogravimetry and DTA, 129 X-ray diffraction patterns, 140
461
Calcium sulphate phascs (scc idso anhydritc; gypsum; hcmihydratc) attack on concrete. 399 cficts on cement hydration, 231-237 eficts on slag hydration, 290 y-CaSO, ('soluble anhydritc') data, 186-187 effect of heating, 54 effect on cement hvdration. 234 formation during clinker grinding, 92 Calorimetry aluminate phase, 194 at low temperatures, 265, 403 calcium aluminate cements. 319 calcium silicates. 160 cement or calcium silicates with accelerators. 358 Portland cement, 226-227 Capillaries, and movement of watcr during freezing, 403 Capillary pores, 247-25 1, 254-256 and modulus of elasticity, 270 and permeability, 273-274 and strength, 266. 268 Capillary stress, 271-272 Carbon dioxide. aggressive, 404-405 Carbonates, and Portland cement hydration, 3 12, 361-362 Carbonation avoidance in small samples. 124 of cement paste in concrete, 383-386 of hydrated aluminate phases, 168. 180, 185, 200 Carbonation shrinkage, 386 Cement based materials, very high strength, 374-376 Cemen t(s) alinite, 343-344 autoclaved, 365-37 1 calcium aluminate, 3 16-334 composite, 276-3 15 expansive, 335-339 high in belite and C,A& 341-343 hydraulic, xv jet, 339-341 latent hydraulic, 277 low energy, 341-344 microsilica, 305-308
462
Index
Ccmcnt(s) ( w t r t . ) oilwell, 371-374 pfa (Class F fly ash). 290-301 pozzolanic. 302-305 rapid setting. 316, 333, 339--341. 361362 regulated sct. 339-34 1 self stressing, 335-336 shrinkage compensated. 335-336 slag. 277-290 supersulphatcd, 290 very high strength, 374-376 with Class C fly ash. 308-31 1 with limestone or other fillers. 312 with rice husk ash or other wastes, 31 1 Ccnospheres. 29 1. 293 Chemical attack on concrete, 405 Chemical shrinkage, 238. 250 Chemically bound watcr, scc bound water Chloride ion and corrosion. 383-384, 386-388 as accelerator or retarder. 357-361, 372-373 in cement making, 74, 76, 94, 343344 in cement paste or concrete, 360-361, 386-387 Chromium substitution in clinkcr phases, 121 Ciment Fondu (see also calcium aluminate cements), 3 16-334 Clay minerals, reactions in ccmcnt making, 60-61, 64, 73-74 Clays as raw matcrials for cement manuFacture, 65-66 heated, as pozzolanic matcrials, 302 Clinker formation in pure systems Ca0-A120,-Fe20,-SiO,, 45-48 CaO-Al,O,-SO,, 3941 Clinker, Portland cement. see Portland cement clinker Cohesion point. 223, 233 Composite cements, 276-3 15 and alkali silica reaction, 394-396 definition, 276 hydration at temperatures up to IOOaC, 363
Composite ccmcnts ( c , o t r r . ) in concrctc, 38 1--382 pore structure and physical propcrtics of pastcs. 3 12-3 15 Concrete ccnicnt paste in. 377-383 durability, 383408 refractory. 3 16. 334 structural variations near exposed surfaces. 382-383 Condensed silica f1111ic. scc microsilica Conductivity. clcctrical calcium aluniinatc ccmcnt pastcs. 319, 321 Portland ccmcnt pastcs, 228 Connectivity, of silicate tctrnhidra. 139 Consistonictcr. high tcnipcraturc. high prcssurc, 372 Convcrsion. in calcium aluniinatc cement pastcs. 320. 328--330 Coolcrs. in Portland ccment clinker making, 70 Cooling of Portland ccnicnt clinkcr reactions during, 84-9 1 studied by light microscopy. 104 Corrosion of concrctc rcinfoscenicnt. 383-384, 387-388 Creep, 272-273 Cristobalitc, 61, 73, 75 Curing, definition, 123 Cuspidine. 58
d'Arcy's law, 273 D-drying, definition. 131 Deceleratory period. 159-160. 226 Defects, and reactivities of clinker phases. 121-122 Deformation of cement pastcs. 269273, 314 Degree of hydration, ultimate. at elevated temperatures. 363-364 De-icing salts, and damage to concrete. 403, 405 Densities (see also under individual phases) Portland cement phases and hydration products. 220 Deposits, in cement kilns or preheaters. 76-77. 81
Index
Design life. of concrete. 383 Dcspujolsitc. 180 Diatomaceous earth. 302 Dicalcium krrite (see also ferrite (phase)). 28-29. 4 1 4 2 . 48-50 u-Dicalcium silicatc hydrate. 364-365. 367-369 Dicalcium silicatc pastcs. see calcium silicate pastcs Dicalcium silicate polylnorphs (see also belite) data. structures, thermal behaviour, 15-23 dusting. 16 enthalpy changes on formation. 6465 enthalpy changes on hydration. 157. 232 equilibria. 15. 33--34. 39. 43-50. 57. 95 in calcium aluminatc cements. 318 modulated structures, 21 preparation, 59 stabilization of P-polymorph. 16, 21 thermal behaviour, 15, 22 Dielectric constants of cement pastcs, 228. 319 Differential thermal analysis AFm phases. 177 AFt phases, 181 CAH,,. 184 C-S-H(1) and similar materials, 147 calcium hydroxide, 129 hydrogarnet phases, 183 Portland cement clinker phases, 15, 22-23 Portland cement pastcs. 200-201. 364 Diffusion in Portland cement pastes. 274--275 of chloride ion in concretes. 386-387 Diopside, 49 Disjoining pressure, 253, 271 Dolomite aggregates, and expansion in concrete, 407408 in cement making, 72, 103 Dreierkette. 144-145, 149 Dry process. 66-70 Drying of C-S-H, effect on silicate anion structure, 140. 147
463
DSP matcrials. 374375 Duplex films, 377-379. 382 Durability ofconcrctc. general aspects. 383 Dust. formation in cclncnt kiln. 70. X I Early product calciuni silicatc pastcs or suspensions. 136-137. 154-157. 162- 166 Portland ccmcnt pastcs. 22 1L223. 228 ~ C1 I.S, '~.' ICI~Y modulus . of. 269 -270. 3 14 Electron diffraction alitc, 13 C,S surlhcc structures. I62 C-S-H(1). hydrothermally prepared. 370 C-S-H(II), 148 calcium silicatc pastcs, 140. 15 l pfa ccmcnt pastes. 296-297 Electron probc microanalysis, see X-ray microanalysis Electron spectroscopy for chemical analysis (ESCA) aluminatc phasc. 194 calcium silicatc pastcs. 162. 166 ferrite phase, 196 Ellestadite, hydroxyl. 74 Enstatitc. 49 Enthalpy changes. see thcrmochcmistry Equilibria (see also under systcm an? phascs concerned) in aqueous solutions, method of calculation, 323-324, 405 Etching, of clinker Ibr microscopy. 101 Ethylene diaminc tetraacetic acid. efict on cement hydration. 347--348 Ettringitc (see also AFt phascs) decomposition by magnesium ion. 399 effect of high temperatures. 181. 19 1. 364 from monosulphatc-calcite reaction, 407 in concrete attacked by sulphates. 397-398, 400 in expansive cements, 335-339 in Portland cement pastes. see under AFt phases in rapidly hardening cements, 339-341
464
Index
Ettringite ( m r r . ) in steam cured products, 402 in supersulphatcd ccmcnts, 290 morphology, 178. 195. 221-224. 338. 400 preparation, 193 quantitative determination, 18 1. 208 Evaporable water. 247 Expansion (see also alkali silica rcaction, sulphatc attack. cxpansivc cements) from excessive gypsum content in cement, 235 in concrete containing dolomitc aggregatcs. 407 Expansive ccments, 335-339 Extraction methods for calcium silicate o r cement pastes. I28 for unreacted Portland cement or clinker, l l 1-1 12 Failure, mechanisms of, 268-269 False set. 233-234 Fatty acid salts, as alr entraining agents, 351 Fe,O, (oxide component), in Portland cement hydration products, 215 Feldman-Sereda model, 252-253 Felspars, reaction with calcium hydroxide and water, 408 Feret's law, 265 Ferrite (phase) colour. 32 compositions in clinkcrs. 10-1 1, 3031 data, structure, 28-32 effect of gypsum on hydration kinetics, 233 electrical and magnetic properties, 32 enthalpy changes on formation, 64 enthalpy changes on hydration, 197198 equilibria, 41-50 factors affecting reactivity, 121-122 formation in cement making, 60-6 1, 74, 84-89 hydration in Portland cement pastes, 2 15220. 225-226, 233
Fcrritc (ph:isc) ( c ~ t ~ l . ) in pul.c systems. 196 197 ionic substitutions, 28 3 I present in calcium aluminarc ccmcnts. 318 -319 X-ray powder patterns. 27. 32. 453 zoning, 43 Fine powders. clkct on ccmcnr hydration kinetics. 3 3 . 3 12 Fire dmxigc. 408 Flash set, 233 Floc structurc. 245-246 clfcct of water rcduccrs and supcrplasticizers. 354 357 Fl~ioridcph:iscs in Portl;ind ccmcnt clinker formation. 56 58. 76 77. 94-95 Fluorosilicatcs. :is Huxcs in ccmcnr making. 58 Fluxcs. in Portland ccmcnt making. 92-93, 341 Fly ash Class C. 3 0 8 3 1 1 Class F. scc pulvcrircd fucl ;is11 Foshagitc, 370 Fracture tncchanics. 269 Frcc limc (calci~tmoxidc) data, structurc. 33 determination. 1 10. 1 12 in cement making. 60--01. 74 75. 79-83 in expansive ccmcnts. 337 microscopic observation in clinkcr. 103 Free watcr porosity. 219. 255 comparison with mcrcury poros~tics. 262-263 in pastes of composite cclnents. 289. 301. 374 Freeze-thaw damage. 35 1. 402-403 Fresh pastcs calcium aluminatc cements. 325 Portland ccmcnt, 243-246 effects of watcr reducers and superplasticizers, 352-357 Friedel's salt, see C,A.CaCI2.H, ,, Fuels. for cement making. 66 Gaize, 302
Index
Garnet, 39, 181-183 Gases, diffusion in Portland ccmcnt pastes, 274-275 Gehlenite, 38-39. 50-5 I , 3 1 X Gchlenitc hydrate, scc C2ASH, Gel cement, 2 1 3--2 16, 247 formed in early pcriod of hydration. 221-223. 228. 245 -240 formed in alkali silica reaction, 388 394 Gel pores. 247-255, 260. 262 Gel/space ratio, 266 Gel water, 13 1. 247-252 Geothermal well cementing. 372. 374 Gibbsite (see also aluminium hydroxide), 184, 324, 326 Glaserite. 16, 54 Glass in Class C fly ash. 308-3 1 1 in ground granulated blastfurnacc slag, 277-282 in microsilica. 305-306 in natural pozzolanas, 299, 302 in Portland cement clinkers. 85 86 in pulverizcd fuel ash (Class F fly ash), 291-292 in white cements, 86 Glass fibre reinforced ccmcnts, dcterioration in moist cnvironnicnts. 408 Granulated blastfurnace slag (see blastfurnace slag, slag cements; supersulphated cemcnts) Grinding aids, 352 Grossular, 39. 18 1-183 Ground granulated blastfurnacc slag. see blastrurnace slag; slag cements: supersulphated cemcnts Guniting, 362 Gypsum attack on concrete, 399 behaviour on grinding and storage of cement, 92 crystal and other data, 186-1 87 effects on cement hydration, 231 equilibria, 188-1 92, 228 formation on sulphate attack, 397398 optimum, 234-236 reactions with calcium aluminate cements. 333
465
Gypsum (co/r/.) secondary. 233 Gyrolitc, 369. 374
Hadley grains, 225 Hardening calcium aluminatc cements. 326-330 definition. relation to hydration rcactions 123 Hatruritc. 15 Heat evolution (see also calorimetry) calcium aluminatc ccmcnts. 3 16 modelling. 24 1-242 Portland cements. 3, 230-23 1 Heavy metals. and Portland cement making, 77. 95 Hcmihydrate effect of admixtures on hydration, 3 59 effects on cement hydration, 234 formation during clinkcr grinding, 92. 233, 236 solubility, structure. data. 186-188 Hibschitc. 181 High alumina cemcnts, see calcium aluminate cements High temperature phases, laboratory preparation, 58-59 High temperatures effect o n Portland cement hydration, 362-365 in autoclave, 365-371 in oilwell cementing, 373-374 Hillebrandite, 369 Hot pressed cements, 375 Humidity, effect on cement hydration kinetics, 238 Hydration definition, 123 reactions and products, see under relevant type of cement termination of, 124-1 25 Hydraulic cements, xv latent, definition, 276 Hydrocalumite, 175 Hydrogarnet phases (see also C,AH,) compositions, data, structures, 18 1I83
466
Index
Hydrogarnct phascs ( c , o ~ l . ) lorma tion from aluminate or ferrite phasc. 194, 197-198 from pfa ccmcnts, 296-297 from Portland cements, 200-20 1, 204. 215-216 from pozzolanic ccmcnts, 304 in autoclave processes. 369 Hydrogrossular, 18 1 Hydrotalcitc-type phases compositions, data, structures, 185 formation from Portland cement, 201, 204, 2 16 from slag cements. 282. 284-285 through sulphatc attack. 400 Hydroxyl-ellestadite. 74 Ice crystal formation. and frcczc thaw damage. 402--403 Image analysis aggregate-ccmcnt paste interface, 379-380 calcium hydroxide in pastes, 128 Portland cement or clinker. 108 lln bibition in alkali silica reaction. 393-394 in expansive cements, 338-339 Induction period calcium aluminate cements, 321 -322, 325 calcium silicate pastes. 159-164, 166 Portland cement pastcs. 226 lnfra red absorption spectra AFni phases. 177 AFt phascs. 181 calcium aluminate ccmcnts. 3 19 calcium silicate pastes, 142 hydrogarnet phascs. 183 Portland cement pastcs, 201-202 unhydrated Portland cement, 113, 201 Inner product definition, calcium silicate pastes, 133 Portland cement pastes, 203, 222, 224-225 Inorganic salts, effect of on calcium aluminate cement hydration, 330-33 1
Inorganic salts (c,otr~.) on calcium silicate o r Portland ccmcnt hydration. 357-362 on hcmihydratc or nnhydritc hydration, 359 Interfacial zone. ccmcnt paste and aggregate. 377-383 Intcrground cements. dctini~ion.'76 Interstitial material in Portland ccmcnt clinkers composition, 86-87 cft'ccts of cooling rntc. 87-88 Ionic substitutions, and rcactivitics of clinker phascs. 121-1 22 Ions. diffusion in Portland cement pastes. 274-275 Iron(ll) oxidc. scc wiistitc Iron(lll) oxidc or hydroxide, as product from ferrite phasc. 196-197, 225 Jaf'citc, 369 Jnsmunditc. 343 Jcnnitc. data, structure. 142 145 KC,,S, ,, non-cxistcncc of. 2 l Katoite, 181 Kilchoanitc. 34 Kiln rings and deposits, 76-77 Kinetics oC hydration ol' calcium silicates. 159-166 of Portland ccmcnt, 237- 241 KOSH rcagcnt ( K O H a n d sucrose). 111-112 Larnitc, 23 Laser granulomctry, 97-98 Late product in calcium silicate pastes. 136-137 in Portland ccmcnt pastes. 203. 224225 Latent hydraulic cements. definition, 276 Lea and Nurse method, 98 Leaching, of concrete by water, 403405 Lead salts, and Portland cement hydration, 361
Index
Lcpol process, 7 1 Light microscopy calcium silicate pastes. 133 Portland cement clinkcr, 101-105 Portland cement pastes. 203 Lignosulphonates as retarders. 349-350 as water reducers. 352-353 Lime, free, see free lime Lime saturation factor. 47. 61-62. 8081. 103 Limestone as addition to Portland cement, 3 12 as raw material for making cement, 65 Linear kinetics. 240 Liquid in formation of Portland ccmcnt clinker composition. 87 physical properties. 8 I. 92-93 quantity, 77-79 solidification, 8 4 9 1 Lithium salts, and calcium aluminate cements, 33 1 Litre weight, 96 Low temperatures. effects on Portland cement hydration. 365 Lurgi process, residues from, 3 1 1
Macropores. definition. 253 Magnesium aluminium hydroxide hydrate and related phases, 185 Magnesium chloride solutions, conccntrated. attack on concrete, 405 Magnesium compounds, anhydrous, of cement chemical interest, 49 Magnesium hydroxide. see brucite Magnesium oxide. see periclase Magnesium salts, and Portland cement hydration, 361 Magnesium silicate. hydrated. from sulphate attack, 400 Magnesium sulphate solutions, attack on concrete. 397, 3 9 9 4 0 0 M a p cracking, 388 Mass balance clinkers, 1 1 6 1 17 FA cement pastes, 300
467
Mass balance (c.otlt.) Portland cement pastes, 2 16-22 1 during early hydration, 228 slag cement pastes, 288 Maturity functions, 362 Maycnite, 38 M D F (Macro Defect Free) cements. 375-376 Mcixncritc, data, structurc, 185 Melamine formaldehyde sulphonatc polymers, 353-354 Mclilitc structurc, 38, 49 Mercury intrusion porosimetry composite cement pastes, 3 13-3 14 gencral, Portland ccmcnt pastcs, 26 1263 Mercury porosity. 262-263, 3 13-3 14 Merwinitc, 49 Mesopores, definition, 253 Metajennite. 145, 152 Metal cations, attack o n concrete. 405 Methanol, reactions with C,S o r cement pastes, 124-1 25 MgO (as oxide component; see also periclase) in compounds, see under names in Portland cement, behaviour o n hydration, 21 5-216 in Portland cement making, 78-79, 87-89, 93-94 Micropores, definition, 253 Microscopy, see light microscopy; scanning electron microscopy; transmission electron microscopy Microsilica (condensed silica fume) formation, composition, physical properties, 305-306 in very high strength materials. 374375 Microsilica cements cement-aggregate bond, 38 1 concrete properties, 305-306 hydration chemistry, 3 0 6 3 0 8 pore structures, 3 13-3 14 Microstructure calcium aluminate cement pastes, 326-327 calcium silicate pastes, 133-1 37 concrete near exposed surfaces, 382-383
468
Index
Microstructure ( c . o t l t . ) dcvclopment in cement pastcs, modelling. 241-242 paste-aggregate interfaces, 377-379 paste-glass interfaces, 377-378. 408 paste-metal interfaces. 382 pfa cement pastes, 296 Portland cement clinker, 101-108 Portland cement (ground), 107-108 Portland cement pastes, 202-204. 22 1-225 after carbonation, 385 after sulphate attack. 397 slag cement pastes. 282 Middle product calcium silicate pastes. 136-1 37 Portland cement pastes, 223-224 Mineral additions, definition, 276 Mineralizers, in Portland cement making, 93 Modelling, mathematical, of Portland cement hydration, 241-242 Modulus of elasticity composite cement pastes, 314 Portland cement pastes, 269-270 Moler, 302 Molybdate method (see also silicate anion structure), 137 Monosulphate (monosulphoaluminate), see C,ASH, phases as hydration product of Portland cement, see under AFm phases Monticellite, 49 Mortars, failure through thaumasite formation, 40 1-402 Mossbauer spectra C-S-H preparations containing Fe3+,214 calcium aluminate cements, 319 ferrite phase hydration, 196-197 Munich model. 253 NC,A3, non-existence of, 25 Nagelschmidtite, 23 Naphthalene formaldehyde sulphonate polymers, 353-354 Neutron diffraction calcium aluminate hydration, 319 porosity determination, 264
Newton's law (rhcology), 243-244 Nitrogen sorption isotherms, 258-261 Nodulization, in clinker formation. 81 Nomenclature, cement chenlical. 4 Non evaporablc water and Powers-Brownyard model. 247-25 1 definition, 131 in calcium silicate pastcs. 131 in cement pastes with Class C fly ash. 310 in microsilica cement pastes, 306-307 in pfa cement pastes, 299. 301 in Portland cement pastcs, 206. 219. 247-25 1 in slag cement pastcs, 287. 289 in supersulphated cement pastes. 2%) specific volume, 250 Nuclear magnetic resonance, "AI alitc hydration products, aluminous C-S-H, 2 14 aluminous tobermorite. 369 CAH,,, 184 C,AH,, 174 calcium aluminate hydration. 319 Nuclear magnetic resonance, proton, 264 Nuclear magnetic resonance, 'Si (see also silicate anion structurc) calcium silicate pastes, etc., 137, 146147, 162, 360 Portland cement pastes, 2 13 tobermorite, 143, 370
Oilwell cementing, 371-374 Okenite, and alkali silica reaction, 393 Ono's method (light microscopy). 105 Opal, as aggregate, 390 Organic compounds, attack on concrete, 405 Outer product calcium silicate uastes. 133 definition, Portland cement pastes. 204, 222 Oxalic acid, effect on C,S hydration. 350 Oyelite, 152
Index
P,O, (oxide component) in Portland cement making, 94-95 Parabolic kinetics, 240 Parawollastonite, 33 Particle size distribution of cement, 96-99 effect on hydration kinetics, 99-100, 239-24 1 Passivation, of steel, 387 Paste (see also under starting material, e.g. calcium silicate pastes. Portland cement pastes) definition, 123 Pelletized blastfurnace slag (see also blastfurnace slag; slag ccmcnts; supersulphated cements), 277 Periclase (magnesium oxide) data, structure, equilibria, 48-51 in expansive cements, 337 in Portland cement clinkers, 2, 88, 94, 103 Permeability composite cement pastes, 3 13-3 15 modelling for cement pastes, 241-242 Portland cement pastes, 273-274 Pessimum composition, 390, 394 Pfa, see pulverized fucl ash Phase composition, potential. 63 Phase equilibria. high temperature. 33-58. 95, 318 Phase Q (C,,A, ,M,S, approx.; sec also pleochroitc), 49-5 1 Phase T (C, ,M,,,S approx.). 49 Phase X (hydration product of Portland cement), 201. 364 Phenolphthalein test, for carbonation. 384-386 Phosphates and Portland cement hydration, 361 and Portland cement making, 94-95 Physical attack on concrete, 402403 Pipette, Andreason. 97 Pleochroite, 51, 318 Plerospheres, 29 1 Plombierite, 142, 152 Point counting, 103 Polymer concrete, 375 Pore size distributions clinkers, 96 composite cement pastes, 3 12-3 14
469
Pore size distributions ( c o t l t . ) Portland ccmcnt pastes. 260 - 265 Pore solutions and alkali silica reaction. 389. 393. 395-396 C,S pastes, 156 hydrating calcium aluminatc cemcnts. 320-325 Portland ccmcnt pastes, 227-230 Pore structurcs composite ccnicnt pastes. 3 12-3 15 and sulphatc attack. 401 Portland ccnicnt pastcs. 254 -265 efkct of cul-ingtcmperaturc. 363.365 Porcs and porosities. definitions capillary, gel, 247 frcc water, 256 mercury. 262 micro, mcso, macro, 253 total. total water, 255 Porosity calculated, 2 19, 22 I , 254 -256, 289, 301. 314 cxperimcntally dclermincd, 255-258. 261-265, 312-314 of 'cement gcl' (Powers- browny yard). 247, 251 Porsal cemcnt, 342 Portland ccnicnt (see also Portland ccnicnt clinkcr) chemical analysis. 100-101 constituent phascs, 1-2 depth of rcaction as function of time. 99- 1 00 high carly strength, 3 hydration, 199-242 at high temperatures, 362-365. 373-374 at low tcmpcraturcs. 365. 374 in autoclave Drocesscs, 365-371 infra red and Ranian spcctroscopy, 113 low heat, 3 mixtures with calcium aluminatc cements, 332-333 moderate heat of hardening, 3 oilwell, 372 particle size distribution, 96-99 phase compositions of particle size fractions, 99
470
Index
Portland cement ( c o t l r . ) phase distributions within grains. 106-1 08 rapid hardening, 3 reactions during storage. 7 1 . 9 1 - 92 specific surfacc area. 98-99 specifications. 2 sulphate resisting, 3. 10-1 1 , 31, 400 thermogravimetry, 1 12-1 13 types of, 2 white, 3, 10-1 1 , 27. 83. 84, 86. 87 X-ray powder diffraction pattcrns. 108-1 1 1 Portland cement clinker calculation of phasc composition, 62-63. 113-1 19 colour. 32. 96 density. 96 effects of minor components. 92-95 general description. 96 light microscopy, 101-105 manufacture. 65-7 1 cffcct o f reducing conditions. 84. 104 enthalpy changes in formation, 63-65 grinding, 70-7 1 , 9 1-92 raw materials. 65-66 reactions during formation. 60-6 1. 7 1-92 physical properties. porc structure. 96 quantitative phasc determination, 113-1 19 reactivities of individual phascs, I I9 122 scannlng clectron microscopy, 105-1 08 typical compositions of phascs. 10-1 1. 114 Portland cement pastes calorimetry, 226-227 compositions of constituent phases, 209-2 19 determination of unreactcd clinker phases, 204-206 effects of admixtures. 345-362 enthalpy changes during hydration, 230-232 fresh, 243-246 kinetics of hydration, 231, 235-241 -
Portland cement pastes ( c . o r ~ ~ . ) microstructurc, 202-204, 22 1-225 models of paste S I ~ L I C ~ L I246~ C . 253 phasc compositions. 199--204. 307 209. 21 6 2 2 1 porc structure. 254 265 relations to physicd properties. 265-275 setting, 23 1 2 3 4 volume pcrccntoges of phases. 219. 22 1 Portlanditc, scc calcium hydroxide Potassium nluminatc. in ccmcnt clinkcrs. 54 I'otassii~m calcium sulphatc. scc calcium langcinitc hydrated. see syngcnitc Potussium iron sulphidc. in rcduccd clinkers. 84 Potassium sodium sulphatc. scc :~phthitalitc Potassium sulphatc (arcmite). data. cquilibria. 52-56 Potassium sulphidc. in rcduccd clinkcrs. 84 I'owcrs-Brownynrd model. 246-251 Pozzolanas and pozrolanic ccmcnts. 299-305 resistance to sea water, 406 Pozzolanic mntcrials. definition. 276 Pouolanic rcaction, 298 in autoclave proccsscs. 367- 37 1 similarity to alkali silica rcaction. 392-393 I'rccalcincrs, for ccmcnt making. 67--69 Precast concrete products. thaumasitc formation in. 402 Prchcatcrs. for ccmcnt making. 67-68 Preparation in laboratory calcium aluminatc hydrates. etc., 192-193 high temperature phascs. 58-59 hydratcd phases in gcncral. 124-125 Primary air, 69 Primary flue gas desulphurization. residues from. 3 1 1 Protected phascs, 43 Proto C,A, 26, 86 Pscudowoliastonite. 33 -
Index
Pulverizcd fuel ash (pfa; fly ash) and alkali silica rcaction, 395-396 Class C, 29 I , 308-3 10 Class F, 290-292 reactivity. 294--295 Pulvcrizcd fuel ash ccments. 292-301 porc structure and physical properties of pastes, 3 12-3 15 Pyknometry. 257-258 I'yroauritc, 185 Quantitative X-ray diKraction analysis AFt phases, 18 1 calcium silicate pastes. 128-1 29 ccmcnt pastcs, 204-206 Portland cement clinker, 108-1 1 I Quartz reactions in cement making, 60-61, 74-75, 80-8 1 thermal changes. 73 Quick sct, 233 Raman spcctroscopy. of unhydrated cements, 1 13 Rankinite, 33 Rate equations. in cement hydration kinetics, 164, 239-241 Raw feed preparation. in cement making, 67, 103 Reactivities of phases calcium aluminate cements, 319 Portland ccments, 1 19-1 22 Reducing conditions, in making Portland cement clinker, 84, 96, 104 Reference intensity ratio, l I0 Renderings, and thaumasite formation, 402 Rctarders, 345-350 calcium aluminate cements, 330-33 1 oilwell cementing, 372-373 Rheology, 243-246 calcium aluminate cements, 325 effects of water reducers and superplasticizers. 352-357 Rice husk ash, 3 1 1 Rosin-Rammler function. 97-99 Rotary kiln, 67, 69-71
471
Salt scaling, 403 SAM reagent (salicylic acid in methanol), I I 1 Sampling of clinkcr, 101 Santorin Earth, 299. 303 Saturation factor (scc also lime saturation factor), 228 Scanning electron microscopy calcium aluminate cement pastes. 326-327 calcium silicate pastcs, 133-1 37 concrete at aggregate-ccment paste interface. 377-383 attacked by alkali silica rcaction. 389 attacked by carbon dioxide. 385 attacked by sulphatcs, 397 near exposed surfaces. 382--383 fresh pastes, 246 general considerations, 105-106, 136 porosity dctcrmination. 263-264 Portland cement or clinker, 105-108 Portland cement pastes, 202-204, 221-225 Sea water attack calcium aluminate cements, 333 concrete, 406 Secondary air, 69 Secondary ion mass spcctroscopy (SIMS). 162 Sedigraph, X-ray, 97-99 Self desiccation, 250 Separation of phascs in Portland cement, 1 1 1-1 12 Setting calcium aluminate cements, 320, 325 definition, calcium silicate pastes, 123 Portland cement pastes, 23 1-234 Sewage, attack on concrete, 406407 Shales, as raw materials for cement making, 65-66 Shotcreting, 362 Shrinkage carbonation. 386 drying, in Portland cement pastes, 270-272 Sieve analysis, 97 Silica fume, see microsilica
472
Index
Silica ratio (silica modulus), 61, 80-8 1, 103 Silicate anion structure calcium silicate pastes, 137-140, 2 14 with admixtures, 360 C-S-H(I), etc.. 146-148, 214 pfa-C,S pastes, 297-298 Portland cement pastes, 212-21 3 pozzolanic cement pastes, 305 I . l nm tobermorite, 370 1 .lnm tobermorite and jennite, 143145 Silicate anions, species in solution, 156157 'Silicate garden' mechanism, 223 Siliceous aggregates, potentially reactive, 390 Silicocarnotite, 54 "Silicosulphate" (C,S2S) data, equilibria, 52-56 in cement making, 74-76 Sjogrenite, 185 Slag cments (see also blastfurnace slag) hydration chemistry, microstructure, 282-290 pore structures and physical properties, 3 12-3 15 sea water attack, 406 sulphate attack, 401 Slump, 243 Slump loss, 353, 354 SO, (as oxide component; see also individual phases) in clinker formation, 55-56, 94 in Portland cement hydration products, 21 6 Sodium chloride, in oilwell cementing, 372-373 Sodium sulphate, data, equilibria, 52-55 Sodium sulphate solutions, attack on concrete, 396401 Soft water, attack on concrete, 4 0 3 4 0 5 Solubility curves CaO-AI,O,-H,O and related systems, 189-192, 321-324 CaO-Si0,-H,O system, 153-1 56 CaS0,-H20 system, 188 Soluble salts, and damage to concrete, 403
Sorption isotherms, 247, 258-261 Specific surface areas, calcium silicate and cement pastes. 257, 259-260, 264 cements. 98-99 effect on hydration kinetics. 238 Spinel, 49 Spurrite data, structure, equilibria. 58 formation in cement kiln. 74. 76 Steam curing high pressure, 365-37 1 low pressure, 362-365 Stecl corrosion, 383-384, 387-388 interface with paste. 382 Stoichiometry of hydration pfa cement pastes, 298-301 Portland cement pastes, 2 16-22 1 slag cement pastes, 286-289 Stritlingite, see C2ASH, Strength development and pore structure, 265-268. 314 calcium aluminate cements, 326-330 microsilica cements, 306 modelling, 24 1-242 pfa cements, 292-293 Portland cements, 234-237. 265-269 slag cements, 278-279 Strength retrogression, 373 Strontium compounds, and cement making, 94 Sucrose and other organic compounds, as retarders, 345-350 Sulphate attack autoclaved materials, 400 calcium aluminate cements, 3 16. 333 composite cements, 40 1 expansive cements, 336 internal, 399 Portland cements, 396400 supersulphated cements, 400 Sulphate phases in Portland cement clinker effects on hydration, 237 estimation from bulk analysis, 114115 formation. 74. 76. 89-9 1. 94 light microscopy, ' I 3 Sulphate resisting Portland cements, 3
Sulphate resisting Portland cements (con[.) ferrite phase compositions, 10, 1 1, 3 1 mechanism of resistance, 400 Sulphide ion attack on concrete, 407 in reduced clinkers, 84 in slag and slag cements, 278,28 1,284 "Sulphospurrite" (C,S,S), see silicosulphate Sulphur compounds, organic, attack on concrete, 406 Sulphur impregnated cement materials, 375, 376 Sulphuric acid, attack on concrete, 405407 Superplasticizers, 353-357 Supersulphated cements, 290 resistance to sulphate attack, 400 Surkhi, 302 Syngenite data, equilibria, 186-1 88, 228 detection by thermal analysis, 112 formation and effects on cement hydration, 228, 233, 237 formation during cement storage, 92, 93 System(s) CaO-A1203, 34-36 CaO-A1203-Fe,O,, 4 1 4 3 Ca0-AI20,-Fe203-SiO,, 4 3 4 5 , 3 18 under mildly reducing conditions, 3 18 Ca0-A1,03-SiO,, 3 8 4 1 CaO-C2S-C,AF, 4 3 4 4 CaO-C2S-C, ,A,-C2F-Mg0, 48-50 CaO-C2S-C, ,A,-C,AF, 4 3 4 8 CaO-C2S-C3P, 95 CaO-Fe203, 4 1 4 2 Ca0-Mg0-A1203-SiO,, 48, 5 1 Ca0-SiO,, 33-34 containing alkalis, 55 containing CaF,, 57 containing CO, or CaCO,, 58 containing MgO, 48 containing P20,, 95 containing SO, or sulphates, 55-56 System(s), hydrated CaO-A1203-H20, 189-190, 320-324 plus N a 2 0 , 331
System(s), hydrated ( c o n [ . ) Ca0-A1203-Si0,-H,O, 190-19 1 CaO-AI,O,-SO,-H,O, 190-1 92 CaO-Si0,-H,O, 153-1 56 at high temperatures and pressures, 369 plus Na,O or K,O, 158 CaS0,-H,O and related, 188-1 89 Tacharanite, 152 Temperature, effect on cement hydration kinetics, 239 Tertiary air, 70 Tetracalcium aluminoferrite, see ferrite (phase) Tetrahedra, paired and bridging, 149 Thaumasite data, structure, thermal behaviour, 180, 181 from sulphate attack with carbonation, 401-402 preparation, 193 Thenardite, data, equilibria, 52-55 Thermal insulation materials, autoclaved, 366 Thermochemistry aluminate and ferrite phase hydration, 197-198 calcium hydroxide dissolution and dehydroxylation, 127 calcium silicate hydration, 157 clinker formation, 63-65 Portland cement hydration, 230-232 Thermodynamics CaO-A1203-H,O system, 323-324 CaO-SO2-H20 system, 157-1 58 CaS0,-CH-K,SO,-H20 system, 188-189 Thermogravimetry AFm phases, 176 AFt phases, 181 CAH,,, 184 calcium hydroxide, 127-128 calcium silicate pastes, 129 data for cement hydration products, 220 hydrogarnet phases, 183 Portland cement pastes, 207-208 unhydrated cements, 1 12
Thickening time, 372 Tillcyitc, 58 Tobermorite. I . l nnl, 143. 367 3 7 1 , 374 Tobcrnioritc. 1.4 nm. 142-145 "Tobcrmorite gel". 149 Total (water) porosity. 255 Transition elements. and Portland ccmcnt making, 95 Transmission electron microscopy alite, 13 C-S-H preparations, 146. 148 calcium silicate pastcs, 136 pfa cement pastcs, 296-297 Portland ccmcnt pastcs. 204. 22 1. 223 Tricalcium aluminate and substituted modifications, scc aluminatc (phase) Tricalcium silicate (see also alitc) enthalpy change on formation, 64-65 cnthalpy change on hydration. 157 equilibria. 33-34. 38-41. 43 50. 50 57. 95 hydration. 123--165 preparation. 59 surface composition. I62 thermitl stability. 33-34, 88-89 X-ray powder patterns, 13-14. 448 Tricalciuni silicate hydratc. 369 Tricalcium silicate pastcs, scc calcium silicate pastcs Tridymitc. 73. 75 Tricthanolaniinc. and cement hydration. 350 Trimethylsilylation (see also silicate anion structure). 1 3 7 1 38 Truscottite. 369 Tuffs. 299 Undcsigncd product. 204. 209 Units of pressure or strcss, 5 Viscosity, definitions. 244 Void spacing factor. 35 1 Volatiles, circulation in ccmcnt manufacture. 70, 74-77. 83 Volume changes, in concrete on frcczing, 403
Volumetric quantities. calcularion on Powers-Hrownynrd modcl. 2 % ) 25 1 Wagner turbidimctcr, 98 Washburn equation, 261 Waste material utilization composite cements, 276. 3 1 1 low cncrgy ccmcnts. 34 1 --343 Portland ccmcnt, 66 Watcr, catcgorics ol' in calcium silicate pastcs. 130 13 1 in Portland ccmcnt pustcs. 206 207 Watcr/ccmcnt ratio. minimum for complete hydration. 248.-25 1 Watcr contents. scc bound water: non cvaporablc water Watcr rcduccrs, 352-357 calcium aluminatc cements, 33 1 oilwell ccmcnting, 372 Water sorption isotherms. 247. 258 259 Wct proccss. 66 -67. 70L7 I White Portland ccmcnts. 3. 10 1 I . 27. 83. 86, 87 White's reagent, 103 Wollastonik, 33. 75 Workability. 243 effect of plh. 292 -293 effect of water rcduccrs or S L I P C ~ plasticizcrs. 352 -353 Wustitc, 48. 318-319 X-ray microanalysis C,S-pozzolana pastcs. 305 calcium silicate pastcs. 132 clinker phases, 8--9, 21 -22. 27 28. 30-3 1 general considcrations. 132 microsilica ccmcnt pastes. 308 pl'a ccmcnt pastcs, 296-297 Portland ccmcnt pastcs, 209-21 7 slag cement pastes. 284-286 X-ray powder diffraction C-S-H gel, C-S-H(1). C-S-H(II). 141, 146, 148-149 calculated pattcrns, 110, 4 4 7 4 5 3
Index
X-ray powder difyraction ( c ~ ) / l / . ) clinker p h ~ c s .13 14. 30. 23. 26 37. 3 IL32, 447-453 Portlimd ccmcnt pastes, 199 20 1. 204-206 unhydr-atcd I'or~land cement 01clinker. 108--1 I 1 X-ray scaltcring. small angle. 304 Xcrogel. delinilion. 246 Xonotlirc. 369. 374 Young's 111oduIus.see cI;is(i~i~)i
475
Zeolites. as pozzol:rnic marcrialc. 702 305 Zeta potential calcium aluminate cements. 325 Portland cements; elkct ol' superplasticircrs. 355 357 Zinc salts in ccmcnt hydration, 361 in ccmcnt making. 77. 95 Zinc substitution in alitcs, 171 Zoning in alitc. 1 \--I2 in lkrritc phase, 32. 43
